Oxidation of alkylbenzenes with cerium complexes containing a tripodal oxygen ligand

Article abstract of DOI:10.1002/ejic.201402772

The treatment of [Ce(L_OEt)₂(NO₃)₂] (L_OEt^- = [Co(η⁵-C₅H₅){P(O)(OEt)₂}₃]^-) with KMnO₄ in water afforded a diamagnetic purple solid 1, which is tentatively formulated as a Ce^IV permanganate complex, "[Ce(L_OEt)₂(MnO₄)₂]". The Zr^IV analogue, [Zr(L_OEt)₂(MnO₄)₂] (2), has been prepared similarly from [Zr(L_OEt)₂(NO₃)₂] and KMnO₄. The recrystallization of 1 from CH₂Cl₂/hexanes at -18 C led to the isolation of dinuclear [(CeL_OEt)₂(μ-L_OEt′)₂][MnO₄]₂ (3), which contains the dianionic tripodal ligand [L_OEt′]^2- ([(η⁵-C₅H₅)Co{P(O)(OEt)₂}₂{P(O)₂(OEt)}]^2-). The reactions of 1 with K[ReO₄] and [NH₄][OsO₃N] afforded the heterodimetallic complexes [Ce(L_OEt)₂(ReO₄)₂] (4) and [Ce(L_OEt)₂(NOsO₃)₂] (5), respectively. The crystal structures of 3 and 5 have been determined. Freshly prepared 1 can oxidize alkylbenzenes such as toluene, ethylbenzene, and cumene at room temperature to give the corresponding ketone and/or alcohol products. Ce-L_OEt complexes are efficient promoters of the aerobic oxidation of alkylbenzenes by a radical mechanism. For example, cumene in the presence of [Ce(L_OEt)₂(H₂O)₂]Cl (1 mM) in air at 100 C for 10 h afforded a ca. 6:1 mixture of 2-phenyl-2-propanol and acetophenone with a total turnover number of 6810.

Full text of DOI:10.1002/ejic.201402772

FULL PAPER
DOI:10.1002/ejic.201402772
Oxidation of Alkylbenzenes with Cerium Complexes
Containing a Tripodal Oxygen Ligand
Xiao-Yi Yi,[a,b] Guo-Cang Wang,[a] Ho-Fai Ip,[a] Wai-Yeung Wong,[c]
Lizhuang Chen,[d] Herman H. -Y. Sung,[a] Ian D. Williams,[a] and
Wa-Hung Leung*[a]
Keywords: Cerium / Heterometallic complexes / O ligands / Oxidation / Alkylbenzenes
[Co(η5-
(LOEt)2(ReO4)2] (4) and [Ce(LOEt)2(NOsO3)2] (5), respectively.
The crystal structures of 3 and 5 have been determined.
Freshly prepared 1 can oxidize alkylbenzenes such as tolu-
ene, ethylbenzene, and cumene at room temperature to give
the corresponding ketone and/or alcohol products. Ce–LOEt
complexes are efficient promoters of the aerobic oxidation of
alkylbenzenes by a radical mechanism. For example, cu-
–
The treatment of [Ce(LOEt)2(NO3)2] (LOEt
=
C5H5){P(O)(OEt)2}3]–) with KMnO4 in water afforded a dia-
magnetic purple solid 1, which is tentatively formulated as a
CeIV permanganate complex, “[Ce(LOEt)2(MnO4)2]”. The ZrIV
analogue, [Zr(LOEt)2(MnO4)2] (2), has been prepared similarly
from [Zr(LOEt)2(NO3)2] and KMnO4. The recrystallization of 1
from CH2Cl2/hexanes at –18 °C led to the isolation of dinu-
clear [(CeLOEt)2(μ-LOEtЈ)2][MnO4]2 (3), which contains the di-
anionic tripodal ligand [LOEtЈ]2– ([(η5-C5H5)Co{P(O)(OEt)2}2-
mene in the presence of [Ce(LOEt)2(H2O)2]Cl (1 mM) in air
at 100 °C for 10 h afforded a ca. 6:1 mixture of 2-phenyl-2-
propanol and acetophenone with a total turnover number of
6810.
{P(O)2(OEt)}]2–). The reactions of
1 with K[ReO4] and
[NH4][OsO3N] afforded the heterodimetallic complexes [Ce-
undergo self-assembly with oxyanions to give heterometallic
oxo compounds and clusters.[6] The reactions of [Ce(LOEt)2-
(NO3)2] with Na2Cr2O7 and [NH4]6[Mo7O24] afforded
[Ce(LOEt)2(η2-Cr2O7)] and the CeIV/MoVI oxo cluster
[H4(CeLOEt)6Mo9O38], respectively.[7] [Ce(LOEt)2(η2-Cr2O7)]
is a stoichiometric oxidant of alcohols, but it is inactive to-
ward hydrocarbon substrates.[7] As an extension of this
study, we sought to synthesize a CeIV–LOEt permanganate
complex, which we expected to be a stronger oxidizing
agent than the dichromate analogue.
Introduction
Cerium(IV) compounds in oxygen ligand environments
are of interest owing to the widespread applications of ce-
rium(IV) oxides in industrial and environmental cataly-
sis.[1–4] Although the molecular mechanisms of cerium-
based heterogeneous catalysis are not well understood, it is
believed that the facile Ce4+/Ce3+ redox couple, their oxy-
gen storage capacity, and metal–ceria interactions play im-
portant roles in their catalytic functions.[4] To gain insight
into the mechanisms of ceria-based catalysts, we sought to
synthesize heterometallic CeIV–O–M complexes and to in-
vestigate their redox and oxidation chemistry.
The Kläui tripodal ligand [Co(η5-C5H5){P(O)(OEt)2}3]–
(denoted as LOEt hereafter, Scheme 1) can stabilize metal
–
ions in high oxidation states owing to its π-donating abil-
–
ity.[5] Previously, we found that LOEt can stabilize tetrava-
lent metal ions such as Ce4+ ions in aqueous solution.[5]
CeIV–LOEt complexes can hydrolyze phosphate diesters and
–
Scheme 1. Structure of the Kläui tripodal ligand LOEt
.
High-valent heterometallic Ce/Mn complexes are of spe-
cial interest because they are potentially relevant to Mn-
doped ceria materials, which exhibit high activity in wet
chemical oxidation.[8] Recently, CeIV/MnIII and CeIV/MnIV
clusters have been synthesized by the self-assembly of CeIV
ions with MnIII salts or permanganate ions, and their mag-
netic properties and catalytic activities have been investi-
gated.[9] Previously, we synthesized and structurally charac-
terized a heterometallic CeIV permanganate complex con-
taining an imidodiphosphinate ligand, [Ce2{N(iPr2PO)2}6-
[a] Department of Chemistry, The Hong Kong University of
Science and Technology,
Clear Water Bay, Kowloon, Hong Kong, China
E-mail: chleung@ust.hk
http://www-chem.ust.hk/Faculty%20staff/leung/content.htm
[b] School of Chemistry and Chemical Engineering, Central South
University,
Changsha, Hunan 410083, China
[c] Department of Chemistry, The Hong Kong Baptist University,
Waterloo Road, Kowloon Tong, Kowloon, Hong Kong, China
[d] School of Biological and Chemical Engineering, Jiangsu
University of Science and Technology,
Zhenjiang, Jiangsu 212003, China
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(MnO4)2], which shows a weak Ce···O=Mn interaction in [Zr(LOEt)2(NO3)2] with KMnO4 yielded a purple complex,
the solid state.[10] [Ce2{N(iPr2PO)2}6(MnO4)2] can oxidize which has been characterized as [Zr(LOEt)2(MnO4)2] (2).
ethylbenzene to acetophenone at room temperature more The NMR and IR spectra of 2 are very similar to those of
efficiently than free permanganate; this suggests that the 1. Owing to their similar spectroscopic data, we believe that
permanganate moieties are activated by the electrophilic 1 has a similar composition to that of 2, that is, “[Ce(LOEt)2-
CeIV center.[10]
(MnO4)2]”. However, unlike 1, 2 is stable in solution and
We herein describe the synthesis of a reactive CeIV per- cannot oxidize alkylbenzenes. This is probably because the
manganate complex by the reaction of [Ce(LOEt)2(NO3)2] permanganate ions in 1 are coordinated and, therefore, “ac-
with KMnO4 in water. This CeIV permanganate complex, tivated” by the CeIV ions (vide infra), whereas those in 2
tentatively formulated as “[Ce(LOEt)2(MnO4)2]”, can oxid- are non-coordinating. Six-coordinate [Zr(LOEt)2]2+ com-
[11]
ize alkylbenzenes at room temperature. We also found that plexes such as [Zr(LOEt)2](OTs)2
(OTs– = tosylate) have
CeIII–LOEt and CeIV–LOEt complexes can function as ef- been reported previously.
ficient promoters for the aerobic oxidation of alkylbenz-
enes, presumably by a radical mechanism.
Compound 1 decomposes gradually to an unidentified
brown species in CH2Cl2 solution at room temperature. It
is unstable even in the solid state and turned brown slowly
at –18 °C under nitrogen. Therefore, in subsequent studies,
1 was freshly prepared from [Ce(LOEt)2(NO3)2] and KMnO4
and used immediately. The UV/Vis spectrum of 1 in CH2Cl2
showed the characteristic permanganate-based ligand-to-
metal charge transfer (LMCT) band centered at λ ≈ 526 nm.
The intensity of the LMCT band gradually decreased as the
solution changed from purple to brown. Recently, Lau and
co-workers reported that Lewis acids such as BF3 and
[Sc(OTf)3] (OTf–= triflate) induce the intramolecular O–O
coupling of [MnO4]– to give O2.[12] We believe that a similar
pathway is involved in the decomposition of 1 in CH2Cl2
and that the permanganate moieties in 1 are activated
through coordination to the Lewis acidic CeIV center.
The recrystallization of 1 from a saturated CH2Cl2/hex-
anes solution at –18 °C afforded dark brown crystals in
25% yield (with respect to Ce). The crystals were identified
as the dinuclear CeIV complex [{(LOEt)Ce}2(μ-LOEtЈ)2]-
[MnO4]2 (3; [LOEtЈ]2– = [CoCp{P(O)(OEt)2}{P(O)2(OEt)}]2–,
Cp = cyclopentadienyl), which contains two dealkylated
tripodal ligands ([LOEtЈ]2–) and two [MnO4]– anions
Results and Discussion
Reactions of [M(LOEt)2(NO3)2] (M = Ce, Zr) with KMnO4
The syntheses of heterometallic CeIV complexes are sum-
marized in Scheme 2. The treatment of [Ce(LOEt)2(NO3)2]
with KMnO4 in water resulted in the precipitation of a reac-
tive purple solid, 1, which decomposes readily in solution
(vide infra). The IR spectrum of 1 showed the Mn–O band
at ν ≈ 900 cm–1. No N–O bands were found; thus, both
˜
nitrate ligands in [Ce(LOEt)2(NO3)2] have been replaced by
permanganate ions. The 1H NMR spectrum of a freshly
prepared, dried sample of 1 in CDCl3 displayed well-re-
solved signals for the LOEt– ligands and is, therefore, indica-
tive of diamagnetic behavior. When the purple CDCl3 solu-
tion of 1 was left to stand at room temperature, it turned
1
brown gradually, and ill-resolved broad H NMR signals
were observed. The resulting 31P{1H} NMR spectrum
showed a broad singlet at δ = 165 ppm, which is characteris-
tic of CeIII–LOEt complexes {e.g., δ = 164 ppm for [Ce(LOEt)2-
(H2O)2]Cl}.
–
(Scheme 3). The other two MnO4 ligands derived from 1
may decompose by a Ce-induced intramolecular O–O cou-
pling.[12] The 31P{1H} NMR spectrum of 3 showed two res-
onances at δ = 118.7 and 88.0 ppm, which are attributed to
–
2–
the LOEt and L
Ј
OEt
ligands, respectively. Also, two sing-
lets were found at δ = 5.09 and 5.11 ppm for the cyclo-
pentadienyl protons of the two tripodal ligands.
–
2–
The dealkylation of LOEt to give the dianionic L
Ј
OEt
ligand by a metal-assisted Arbuzov reaction and the elimi-
nation of alkyl chloride has been reported. For example, the
treatment of NaLOEt with YbCl3 and ZrCl4 afforded [LO-
EtY]2(μ-LOEtЈ)2 and [Zr(LOEtЈ)2]2, respectively.[13] However,
in the absence of nucleophilic chloride ligands, we believe
that the formation of 3 possibly involves the CeIV-mediated
–
hydrolysis of the P–OEt group in LOEt (to give a P–OH
intermediate and EtOH) instead of the Arbuzov-type reac-
tion. It should be noted that CeIV compounds are very
Scheme 2. Synthesis of heterometallic CeIV and ZrIV complexes. active catalysts for the hydrolysis of phosphate diesters and
DNA.[14]
Reagents: (i) KMnO4, water, 0 °C; (ii) KMnO4, water, room temp.;
(iii) KReO4, water, room temp.; (iv) [NH4][OsO3N], water, room
The structure of the complex cation in 3 is shown in Fig-
temp.
ure 1, and selected bond lengths and bond angles are listed
A ZrIV permanganate complex has been synthesized to in Table 1. The structure consists of two symmetry-related
model the reactive CeIV analogue 1. The treatment of eight-coordinate [Ce(LOEt)(LOEtЈ)(H2O)]+ units that are
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synthesized. The treatment of [Ce(LOEt)2(NO3)2] with
KReO4 in water afforded [Ce(LOEt)2(ReO4)2] (4). The
31P{1H} NMR spectrum of 4 in CDCl3 displayed a singlet
at δ = 119.9 ppm. The IR spectrum of 4 showed a ν(Re–O)
band at ν = 869 cm–1, which is lower than that of uncoordi-
˜
nated [ReO4]– (912 cm–1). Unfortunately, we have not been
able to crystallize 4 for structure determination. It seems
likely that 4 is an eight-coordinate CeIV complex containing
–
two η1-ReO4 ligands.
Similarly, the treatment of [Ce(LOEt)2(NO3)2] with
2 equiv. of [NH4][OsO3N] in water led to the precipitation
of an orange solid. Recrystallization from CH2Cl2/Et2O af-
forded orange crystals that were identified as [Ce(LOEt)2-
(NOsO3)2] (5) by X-ray diffraction. The IR spectrum of 5
showed sharp peaks at ν = 887 and 902 cm–1, assignable to
˜
ν(Os–O). By comparison, the Os–O stretches for [NH4]-
[OsO N] are found at ν = 871 and 891 cm–1 and that of
˜
3
[Au(PPh )(NOsO )] occurs at ν = 894 cm–1.[16] The ν(Os–
˜
3
3
Scheme 3. Formation of 3 from 1.
N) band for 5 has not been determined owing to the overlap
with the P=O bands of the LOEt ligand. The molecular
–
linked together through a P=O group of [LOEtЈ]2–. A center
of inversion is located at the center of the complex cation.
The Ce–OP(OEt)2 distances for [LOEtЈ]2– [average 2.344 Å]
structure of 5 is shown in Figure 2. Owing to the disorder
of the nitridoosmate(VIII) ligands, the bond lengths of the
complex have not been analyzed. Nevertheless, the identity
of 5 with two N-bound nitridoosmate(VIII) ligands has
been confirmed by the X-ray diffraction study. To the best
of our knowledge, 5 is the first example of a heterometallic
CeIV/OsVIII complex.
–
are similar to those for LOEt [average 2.340 Å][6] The
PO2(OEt) group in [LOEtЈ]2– binds to the two Ce centers
symmetrically with Ce–O distances of 2.265(4) and
2.234(4) Å. The Ce–OH2 bond [2.488(4) Å] is shorter than
that in the CeIV oxo–aqua complex [Ce(LOEt)2(O)(H2O)]·
Me2CONH2 [2.572(3) Å].[15]
Figure 1. Molecular structure of the complex cation in 3. Hydrogen
atoms are omitted for clarity. Thermal ellipsoids are drawn at the
30% probability level. Symmetry transformations used to generate
equivalent atoms: A = –x + 1, –y + 1, –z + 1.
Table 1. Selected bond lengths [Å] and angles [°] for 3.
Figure 2. Molecular structure of 5. Hydrogen atoms are omitted
for clarity.
Ce(1)–O(2)A
Ce(1)–O(8)
Ce(1)–O(10)
P(1)–O(7)
2.234(4)
2.347(4)
2.488(4)
1.522(4)
Ce(1)–O(7)
Ce(1)–O(9)
P(1)–O(2)
P(1)–O(1)
2.265(4)
2.340(4)
1.521(4)
1.620(5)
P(1)–O(2)–Ce(1)A 162.7(3)
P(2)–O(8)–Ce(1) 138.0(3)
P(1)–O(7)–Ce(1) 141.6(2)
P(3)–O(9)–Ce(1) 137.0(2)
Stoichiometric Oxidation of Alkylbenzenes by 1
Similar to previously reported Lewis acid/KMnO4 sys-
tems,[17] 1 is a stoichiometric oxidant for alkylbenzenes. As
1 is unstable in most organic solvents, the oxidation of neat
alkylbenzenes with 1 was studied (Table 2). Although 1 is
Heterometallic CeIV/ReVII and CeIV/OsVIII Complexes
To gain insight into the identity of 1, CeIV complexes only slightly soluble in ethylbenzene, stirring a suspension
containing more-stable third-row d0 metal–oxo anions were of 1 in ethylbenzene at room temperature for 2 min resulted
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in a homogeneous purple solution that gradually turned Table 3. Ce-promoted aerobic oxidation of cumene.[a]
brown in 30 min. Gas–liquid chromatography (GLC) analy-
sis revealed that acetophenone was formed as the sole or-
ganic product; no 1-phenylethanol or benzoic acid was de-
tected. The reaction was complete in ca. 2 h. Iodometric
titration indicated that the inorganic product is approxi-
mately seven oxidizing levels below the starting material 1,
Catalyst
Conv. [%]
Yield [%][b]
Total TON
which suggests that the inorganic product is possibly a
CeIII/MnIV complex. As 1 is a seven-electron oxidant and
the conversion of ethylbenzene to acetophenone is a four-
electron process, the yield of acetophenone was calculated
to be 85%. No significant difference in the yield was found
when the oxidation was performed in air. Under the same
conditions, ethylbenzene was oxidized by [nBu4N][MnO4]
or [Ce(LOEt)2(NO3)2] to give acetophenone in less than 5%
yield. Thus, it appears that the permanganate moieties in 1
are activated by the Lewis acidic CeIV center (possibly
through coordination of the Mn=O group to the Ce center).
I
II
1
52
81
77
trace
76
95
10
12
10
trace
43
82
35
69
53
trace
33
13
3250
5800
4480
–
5420
6810
[Ce(LOEt)2(NO3)2]
[Ce(LOEt)(NO3)3]
[nBu4N][MnO4]
[Ce(LOEt)2(NO3)]
[Ce(LOEt)2(H2O)2]Cl
[a] Experimental conditions: cumene (2.25 mL), Ce complex
(1 mm), air, 100 °C, 10 h. [b] Product yields were determined by
NMR spectroscopy after reduction with PPh3. Small amounts of
α-methylstyrene were also detected.
Table 2. Stoichiometric oxidation of alkylbenzenes by freshly pre-
amounts of products were detected, presumably as a result
of the stoichiometric oxidation of cumene with permanga-
nate. On the other hand, Ce–LOEt complexes are active pro-
moters of cumene oxidation. For example, heating cumene
with catalytic [Ce(LOEt)2(NO3)2] at 100 °C afforded a ca.
1:6 mixture of 2-phenyl-2-propanol and acetophenone with
81% conversion and a total TON of 5800. Similar conver-
sions and TONs were found for [CeIV(LOEt)(NO3)3] and
[CeIII(LOEt)2(NO3)]. Of all the Ce complexes studied, the
CeIII complex [Ce(LOEt)2(H2O)2]Cl (conversion = 95%,
total TON = 6810) is the most-active catalyst. This result
pared 1.[a]
Alkylbenzene
Products (yield, %)[b]
Toluene
Ethylbenzene
Cumene
benzaldehyde (31)
acetophenone (85)
2-phenyl-2-propanol (35), benzaldehyde (36)
Diphenylmethane benzophenone (75)
[a] Experimental conditions: freshly prepared 1 (0.011 m), neat alk-
ylbenzene, 23 °C, 2 h, under nitrogen. [b] Yields were determined
by GLC with the assumption that 1 is a seven-electron oxidant
(vide infra).
Similarly, stirring a suspension of 1 in toluene resulted indicates that the Ce center instead of the Mn center is
in a brown solution in ca. 3 h. GLC analysis showed that responsible for the catalytic activity of 1 in the aerobic oxi-
benzaldehyde was formed in 31% yield; no benzoic acid was dation.
detected. The yield of toluene oxidation is lower than that
for ethylbenzene apparently because of the stronger benz-
ylic C–H bonds [the bond dissociation energies (BDEs) for
toluene and ethylbenzene are 90 and 85 kcal/mol, respec-
tively].[18] Unlike those of ethylbenzene and toluene, the oxi-
dation of cumene by 1 afforded 2-phenyl-2-propanol (in
35% yield, assuming two-electron oxidation) along with
benzaldehyde (36%). The oxidation of diphenylmethane
gave benzophenone in 75% yield.
Table 4. Aerobic oxidation of alkylbenzenes with [(LOEt)2Ce(H2O)2]-
Cl.[a]
Alkylbenzene
Toluene[b]
Products (yield, %)
Total TON
81
benzyl alcohol (0.5)
benzaldehyde (0.6)
Ethylbenzene[b] 1-phenylethanol (12)
acetophenone (13)
1910
6810
Cumene[c]
2-phenyl-2-propanol (82)
acetophenone (13)
[a] Conditions: alkylbenzene (2.25 mL), [Ce] = 1 mm, air, 10 h,
100 °C. [b] Product yields were determined by GLC after reduction
with PPh3. [c] Product yields were determined by H NMR spec-
troscopy after reduction with PPh3.
1
Ce-Promoted Aerobic Oxidation of Alkylbenzenes
Although 1 cannot catalyze the oxidation of alkylbenz-
enes with molecular oxygen at room temperature, it was
As [Ce(LOEt)2(H2O)2]Cl appears to be an efficient initia-
found to be an efficient initiator for the autoxidation of tor for cumene autoxidation, the aerobic oxidation of other
alkylbenzenes at higher temperatures. For example, cumene alkylbenzenes with this CeIII complex was studied (Table 4).
in the presence of 1 mm of 1 at 100 °C afforded a mixture Heating ethylbenzene at 100 °C in the presence of 1 mm of
of 2-phenyl-2-propanol and acetophenone with a conver- [Ce(LOEt)2(H2O)2]Cl afforded a ca. 1:1 mixture of 1-phen-
sion of 52% and a total turnover number (TON) of 3250 ylethanol and acetophenone with a total TON of 1910. A
(Table 3).
smaller TON (81) was found for toluene because of its
To find out whether the Ce or Mn center is responsible stronger benzyl C–H bonds.[17]
for the catalytic activity of 1, the aerobic oxidation of cu-
The metal-catalyzed oxidation of benzylic C–H bonds
mene with various Ce–LOEt complexes and [nBu4N][MnO4] with molecular oxygen has been studied extensively. Gen-
was examined (Table 4). [nBu4N][MnO4] is not capable of erally, free radical mechanisms involving peroxy radicals
catalyzing the aerobic oxidation of cumene; only trace and hydroperoxide intermediates have been proposed.[19]
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We believe that similar pathways are involved in the Ce- ficiently than free [MnO4]– ions; this suggests that the per-
promoted aerobic oxidation of alkylbenzenes such as cu- manganate ions are activated by the CeIV center (possibly
mene (Scheme 4). The oxidation of cumene by a CeIV spe- through coordination to the Ce atom). Ce–LOEt complexes,
cies followed by reaction with oxygen gives a peroxy radical. notably the CeIII complex [Ce(LOEt)2(H2O)2]Cl, can pro-
Subsequent hydrogen atom abstraction from cumene yields mote the aerobic oxidation of alkylbenzenes such as ethyl-
cumene hydroperoxide. The reduction of the hydroperoxide benzene and cumene, presumably by a free radical pathway.
with a CeIII species gives an oxy radical, which is finally
converted to 2-phenyl-2-propanol or acetophenone.[20] Con-
sistent with this radical mechanism, the Ce-based aerobic
Experimental Section
oxidation of cumene was completely quenched by radical
General: NMR spectra were recorded with a Bruker ARX 300 spec-
traps such as 2,6-di-tert-butyl-4-methylphenol and (2,2,6,6- trometer operating at 300 and 121.5 MHz for 1H and 31P NMR
spectroscopy, respectively. Chemical shifts (δ, ppm) are reported
with reference to SiMe4 (1H) and H3PO4 (31P). Infrared spectra
were recorded with a Perkin–Elmer 16 PC FTIR spectrophotome-
ter.
tetramethylpiperidin-1-yl)oxy. The high catalytic activity of
the Ce–LOEt complexes in the aerobic oxidation can be at-
tributed to their facile CeIV/CeIII redox couples (e.g., the
CeIV/CeIII redox couple for [Ce(LOEt)2(NO3)2] in aceto-
nitrile occurs at ca. 0 V vs. Cp2Fe+/0,[21]). It may be noted
that cerium(IV) ammonium nitrate has also been used as
a catalyst for the oxidation of alkylbenzenes with bromate
salts.[22]
Gas chromatography was performed with a Hewlett Packard 5860
chromatograph equipped with a flame ionization detector (FID).
Elemental analyses were performed by Medac Ltd., Surrey, United
Kingdom. Toluene, ethylbenzene, and cumene were washed with
concentrated H2SO4, distilled water, and then 5% aqueous
NaHCO3. They were then dried with Na2SO4, distilled from CaH2
under argon, and degassed by three freeze–pump–thaw cycles be-
fore use. The complexes [Ce(LOEt)2(NO3)2],[6] [Zr(LOEt)2(NO3)2],[23]
(NH4)[OsO3N],[24] and [Ce(LOEt)2(NO3)][25] were synthesized ac-
cording to literature methods. [Ce(LOEt)2(H2O)2]Cl and [Ce(LOEt)2-
(NO3)] were obtained by the reactions of NaLOEt with CeCl3 and
Ce(NO3)3, respectively, in water.[26]
[Ce(LOEt)2(MnO4)2] (1): To
a solution of KMnO4 (26 mg,
0.16 mmol) in water (1 mL) in an ice bath was added [(LOEt)2-
Ce(NO3)2] (60 mg, 0.045 mmol) in water (10 mL), and the mixture
was stirred at 0 °C for 1 min. The purple precipitate 1 was collected,
washed with cold water (0.5 mLϫ3), and dried with a stream of
1
nitrogen, yield 40 mg (61% relative to Ce). H NMR (CDCl3): δ =
1.25 (t, 36 H, CH3), 4.08 (m, 24 H, CH2), 5.12 (s, 10 H, Cp) ppm.
31P{1H} NMR (CDCl3): δ = 120.5 (s) ppm. UV/Vis (CH2Cl2): λ =
526 nm. IR (KBr): ν = 900 [ν(Mn–O)] cm–1. Satisfactory analytical
˜
data could not be obtained owing to the instability of the com-
pound in the solid state.
[Zr(LOEt)2(MnO4)2] (2): To
a solution of KMnO4 (32 mg,
0.2 mmol) in water (4 mL) was added [Zr(LOEt)2(NO3)2] (64 mg,
0.05 mmol) in water (15 mL), and the mixture was stirred at room
temperature for 20 min. The purple precipitate was collected,
washed with cold water (0.5 mLϫ3), and dried in air. Recrystalli-
zation from CH2Cl2/Et2O afforded a purple crystalline solid, yield
Scheme 4. Proposed mechanism for the Ce-promoted aerobic oxi-
dation of cumene.
1
50 mg (71%). H NMR (CDCl3): δ = 1.26 (t, 36 H, CH3), 4.07 (m,
24 H, CH2), 5.13 (s, 10 H, Cp) ppm. 31P{1H} NMR (CDCl3): δ =
117.6 (s) ppm. IR (KBr): ν = 900 [ν(Mn–O)] cm–1. C H Co -
˜
34 70
2
Mn2O26P6Zr (1399.71): calcd. C 29.18, H 5.04; found C 29.09, H
5.09.
Conclusions
[(LOEtCe)2(μ-LOEtЈ)2][MnO4]2 (3): A freshly prepared sample of 1
(65 mg) was dissolved in a minimum amount of CH2Cl2 (ca. 1 mL),
and Et2O/hexane (1 mL, 1:1) was added carefully. The mixture was
concentrated to ca. 1 mL and cooled to –18 °C overnight to afford
dark brown needles that were suitable for X-ray analysis, yield
15 mg (25%). 1H NMR (CDCl3): δ = 1.18–1.30 (m, 66 H, CH3),
4.10 (m, 44 H, CH2), 5.09 (s, 10 H, Cp), 5.11 (s, 10 H, Cp) ppm.
We have synthesized heterodimetallic CeIV/MnVII, CeIV/
ReVII, and CeIV/OsVIII complexes by the reactions of [Ce-
(LOEt)2(NO3)2] with the corresponding anionic metal–oxo
complexes in water. The structure of the CeIV/OsVIII com-
plex [Ce(LOEt)2(NOsO3)2] featuring two η1-[OsO3N]–
ligands has been established by X-ray crystallography.
Upon recrystallization from CH2Cl2/hexanes, the CeIV per-
manganate complex, possibly “[Ce(LOEt)2(MnO4)2]”, was
converted to dinuclear [(LOEtCe)2(μ-LOEtЈ)2][MnO4]2, which
contains two bridged dealkylated tripodal ligands. “[Ce-
31P{1H} NMR (CDCl ): δ = 88.0 (s), 118.7 (s) ppm. IR (KBr): ν
˜
3
= 906 [ν(Mn–O)] cm–1. C64H130Ce2Co4Mn2O44P12 (2601.21): calcd.
C 29.55, H 5.04; found C 29.20, H 4.95.
[Ce(LOEt)2(ReO4)2] (4): To
a solution of KReO4 (34 mg,
(LOEt)2(MnO4)2]” can oxidize alkylbenzenes more ef- 0.12 mmol) in water (2 mL) was added [(LOEt)2Ce(NO3)2] (40 mg,
Eur. J. Inorg. Chem. 2014, 6097–6103
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FULL PAPER
0.030 mmol) in water (10 mL), and the mixture was stirred at room
temperature for 10 min. The orange solid was collected, redissolved
in CH2Cl2/Et2O, and dried with anhydrous Na2SO4. Slow evapora-
tion of the solvent in air afforded an orange microcrystalline solid,
yield 37 mg (72%). 1H NMR (CDCl3): δ = 1.29 (t, 36 H, CH3),
4.12 (m, 24 H, OCH2), 5.08 (s, 10 H, Cp) ppm. 31P{1H} NMR
Table 5. Crystallographic data and experimental details for 3 and
5.
3
5
Formula
C66.60H146
Ce2Cl0.40Co4Mn2O46P12
2694.69
13.0067(15)
22.051(3)
18.548(2)
90
-
C34H70-
CeCo2N2O24Os2P6
1727.21
12.0154(4)
12.0154(4)
71.386(5)
90
Fw
a [Å]
b [Å]
c [Å]
(CDCl ): δ = 119.9 (s) ppm. IR (KBr): ν = 869 [ν(Re–O)] cm–1.
˜
3
C34H70CeCo2O26P6Re2·Et2O (1785.26): calcd. C 25.57, H 4.52;
found C 25.27, H, 4.35.
α [°]
[Ce(LOEt)2(NOsO3)2] (5): This complex was synthesized similarly
to 5 by using [NH4][OsO3N] in place of KReO4. Recrystallization
from CH2Cl2/Et2O/hexanes afforded orange crystals that were suit-
able for X-ray analysis, yield 38.5 mg, (75%). 1H NMR (CDCl3): δ
= 1.29 (t, 36 H, CH3), 4.12 (m, 24 H, OCH2), 5.07 (s, 10 H, Cp)
β [°]
97.042(2)
90
5279.7(11)
2
90
120
8925.2(8)
6
γ [°]
V [Å3]
Z
Crystal system
Space group
ρcalcd. [gcm–3]
T [K]
monoclinic
P21/n
1.695
173(2)
1.963
hexagonal
P65
1.928
293(2)
5.784
ppm. 31P{1H} NMR (CDCl ): δ = 121.3 (s) ppm. IR (KBr): ν =
˜
3
819, 887 [ν(Os–O)], 902 [ν(Os–N)] cm–1. C34H70O24CeCo2N2Os2P6·
Et2O (1789.32): calcd. C 25.51, H 4.51, N 1.57; found C 25.43, H
4.38, N 1.54.
μ [mm–1]
F(000)
2749
5076
Stoichiometric Oxidation of Alkylbenzenes with 1: Typically, to alk-
Reflections
Independent refl.
Rint
27443
9110
0.0961
52568
13972
0.0557
ylbenzene (2.25 mL) was added freshly prepared
1 (35 mg,
0.024 mmol), and the mixture was stirred at room temperature un-
der nitrogen in the dark for 2 h, during which the solution changed
from purple to brown. The yields of the organic product(s) were
determined by GLC analysis with bromobenzene as an internal
standard.
R1, wR2 [IϾ2σ(I)]
R1, wR2 (all data)
GoF
0.0495, 0.0859
0.1296, 0.0958
0.961
0.0801, 0.1887
0.14, 0.2176
1.044
Iodometric Titration:[27,28] The Mn oxidation state of the inorganic
product for the oxidation of alkylbenzenes by 1 was determined by
an iodometric method. At the end of reaction, an aliquot (1.0 mL)
of the solution was transferred to a volumetric flask containing
nPr4NI (0.019 g). Glacial acetic acid (1 mL) and CH2Cl2 were
added to bring the total volume to 10 mL. An aliquot of the re-
sulting solution was transferred to a cuvette, and the UV/Vis spec-
Acknowledgments
The financial support from the Hong Kong Research Grants Coun-
cil (project number 602310) and the Hong Kong University of Sci-
ence and Technology is gratefully acknowledged.
–
trum was recorded. The concentration of I3 ions formed was cal-
–
culated with the assumption that ε365(I3 ) = 26200 m–1 cm–1. The
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Published Online: November 12, 2014
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim