Heterometallic cerium(IV) perrhenate, permanganate, and molybdate complexes supported by the imidodiphosphinate ligand [N(i -Pr2PO) 2]-

Article abstract of DOI:10.1021/ic302567e

Heterometallic cerium(IV) perrhenate, permanganate, and molybdate complexes containing the imidodiphosphinate ligand [N(i-Pr₂PO) ₂]^- have been synthesized, and their reactivity was investigated. Treatment of Ce[N(i-Pr₂PO)₂]₃Cl (1) with AgMO₄ (M = Re, Mn) afforded Ce[N(i-Pr₂PO) ₂]₃(ReO₄) (2) or Ce₂[N(i-Pr ₂PO)₂]₆(MnO₄)₂ (3). In the solid state, 3 is composed of a [Ce₂{N(i-Pr₂PO) ₂}₆(MnO₄)]⁺ moiety featuring a weak Ce-OMn interaction [Ce-OMn distance = 2.528(8) ?] and a noncoordinating MnO₄^- counteranion. While 3 is stable in the solid state and acetonitrile solution, it decomposes readily in other organic solvents, such as CH₂Cl₂. 3 can oxidize ethylbenzene to acetophenone at room temperature. Treatment of 1 with AgBF₄, followed by reaction with [n-Bu₄N]₂[MoO₄], afforded [Ce{N(i-Pr ₂PO)₂}₃]₂(μ-MoO₄) (4). Reaction of trans-Ce[N(i-Pr₂PO)₂]₂(NO ₃)₂ (5), which was prepared from (NH₄) ₂Ce(NO₃)₆ and K[N(i-Pr₂PO) ₂], with 2 equiv of [n-Bu₄N][Cp*MoO₃] yielded trans-Ce[N(i-Pr₂PO)₂]₂(Cp* MoO₃)₂ (6). 4 can catalyze the oxidation of methyl phenyl sulfide with tert-butyl hydroperoxide with high selectivity. The crystal structures of complexes 3-6 have been determined.

Full text of DOI:10.1021/ic302567e

Article
pubs.acs.org/IC
Heterometallic Cerium(IV) Perrhenate, Permanganate, and
Molybdate Complexes Supported by the Imidodiphosphinate Ligand
−
[
N(i‑Pr PO) ]
2 2
†
†
,†
‡
†
‡
Guo-Cang Wang, Herman H. Y. Sung, Feng-Rong Dai, Wai-Hang Chiu, Wai-Yeung Wong,
Ian D. Williams, and Wa-Hung Leung*
†
†Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, People’s
Republic of China
‡
Department of Chemistry, Hong Kong Baptist University, Waterloo Road, Kowloon Tong, Hong Kong, People’s Republic of China
*
S Supporting Information
ABSTRACT: Heterometallic cerium(IV) perrhenate, per-
manganate, and molybdate complexes containing the
−
imidodiphosphinate ligand [N(i-Pr PO) ] have been synthe-
2
2
sized, and their reactivity was investigated. Treatment of
Ce[N(i-Pr PO) ] Cl (1) with AgMO (M = Re, Mn) afforded
2
2 3
4
Ce[N(i-Pr PO) ] (ReO ) (2) or Ce [N(i-Pr PO) ] (MnO )
2
2 3
4
2
2
2 6
4 2
(
3). In the solid state, 3 is composed of a [Ce {N(i-
2
+
Pr PO) } (MnO )] moiety featuring a weak Ce−OMn
2
2
6
4
interaction [Ce−OMn distance = 2.528(8) Å] and a
−
noncoordinating MnO counteranion. While 3 is stable in
4
the solid state and acetonitrile solution, it decomposes readily
in other organic solvents, such as CH Cl . 3 can oxidize
2
2
ethylbenzene to acetophenone at room temperature. Treat-
ment of 1 with AgBF , followed by reaction with [n-Bu N] [MoO ], afforded [Ce{N(i-Pr PO) } ] (μ-MoO ) (4). Reaction of
4
4
2
4
2
2
3 2
4
trans-Ce[N(i-Pr PO) ] (NO ) (5), which was prepared from (NH ) Ce(NO ) and K[N(i-Pr PO) ], with 2 equiv of [n-
2
2 2
3 2
4 2
3 6
2
2
Bu N][Cp*MoO ] yielded trans-Ce[N(i-Pr PO) ] (Cp*MoO ) (6). 4 can catalyze the oxidation of methyl phenyl sulfide with
4
3
2
2 2
3 2
tert-butyl hydroperoxide with high selectivity. The crystal structures of complexes 3−6 have been determined.
INTRODUCTION
presence of carboxylic acids and their magnetic and catalytic
properties of these Ce/Mn complexes have been studied. In
one report, it was found that the high oxidation state of the Ce
■
8
Despite the widespread use of Ce(IV)-containing compounds
and materials in organic synthesis and bioinorganic and
1
2
3
,4
and Mn ions (+4) in a CeMn cluster is essential for its catalytic
6
environmental catalysis, the coordination and organometallic
chemistry of tetravalent cerium has not been well-explored due,
in part, to the high oxidizing power of Ce(IV). The functions of
Ce(IV) reagents and catalysts are critically dependent on the
facile Ce(IV/III) redox couple and high Lewis acidity of the
Ce(IV) center. Also, the metal−ceria interaction is believed to
play a role in catalytic performances of ceria-supported
5
activity in aerobic oxidation of alcohols with TEMPO.
Whereas coordination polymers containing the permanganate
13
ion are well-documented, to our knowledge, molecular
Ce(IV)/Mn(VII) complexes have not been isolated to date.
In this work, we synthesized and structurally characterized
heterometallic Ce(IV)-O-M complexes supported by a
1
4
−
3
bidentate imidodiphosphinate ligand, [N(i-Pr PO) ]
2 2
catalysts. Nevertheless, not many studies have been devoted
(
Chart 1), which can be considered as an inorganic analogue
of acetylacetonate.
In contrast with the well-known [N(Ph PO) ] ligand, the
to investigate the influence of the electrophilic, oxidizing
Ce(IV) center on the reactivity of molecular heterometallic
−
5,6
2
2
Ce(IV) complexes.
Relatively few heterobimetallic Ce(IV)-O-M complexes and
coordination chemistry of the more soluble isopropyl analogue
−
14
7
−12
[N(i-Pr PO) ] is not well-developed. In a previous paper,
clusters have been reported in the literature.
include those supported by alkoxide and carboxylate
coligands and Ce(IV)-containing polyoxometalates. We are
particularly interested in heterometallic Ce/Mn complexes
because Mn-doped ceria materials have been shown to exhibit
Example8s
2 2
7
we reported the synthesis of tetravalent Ce-[N(i-Pr PO) ] oxo
2 2
9
and peroxo complexes, demonstrating that the imidodiphos-
phinate can serve as a good spectator ligand for the oxidizing
15
Ce(IV) ion. Herein, we describe the synthesis, structures, and
3
high catalytic activity in wet chemical oxidation. Recently, a
series of Ce(IV)/Mn(III,IV) clusters have been synthesized by
Received: November 23, 2012
Published: February 13, 2013
reactions of cerium ammonium nitrate with KMnO in the
4
©
2013 American Chemical Society
2556
dx.doi.org/10.1021/ic302567e | Inorg. Chem. 2013, 52, 2556−2563

Inorganic Chemistry
Article
−
solution was stirred at room temperature for 1 h. The AgCl formed
was filtered off, and the solvent was removed under reduced pressure.
Chart 1. Structure of [N(i-Pr PO) ]
2
2
Recrystallization from Et O afforded a yellow crystalline solid. Yield:
2
1
6
9 mg (56%). H NMR (C D ): δ 1.19 (m, 36H, (CH ) CH), 1.28
6
6
3 2
3
1
(
m, 36H, (CH ) CH), 2.04 (sept, 12H, J = 7.2 Hz, (CH ) CH). P
3
2
3
2
1
{
H} NMR (C D ): δ 52.38 (s). Anal. Calcd for C H CeN O P Re:
6
6
36 84
3
10 6
C, 35.12; H, 6.88; N, 3.41. Found: C, 35.05; H, 6.99; N, 3.60.
Synthesis of Ce [N(i-Pr PO) ] (MnO ) (3). To a solution of
2
2
2 6
4 2
AgMnO (0.154 mmol) in MeCN (10 mL) was added a solution of 1
4
(
157 mg, 0.154 mmol) in MeCN (10 mL), and the mixture was stirred
reactivity of Ce(IV)-[N(i-Pr PO) ] perrhenate, permanganate,
2
2
at room temperature for 0.5 h and filtered. The filtrate was
concentrated to ca. 5 mL under vacuum and cooled to −18 °C to
give purple crystals that were suitable for X-ray diffraction. Yield: 85
and molybdate complexes featuring Ce(IV)-O-M (M = Re, Mn,
Mo) linkages and their activity in oxidations of ethylbenzene
and organic sulfides.
1
mg (50%). H NMR (CD CN): δ 1.20 (m, 144H, (CH ) CH), 2.01
3
3 2
31
1
(
(
sept, 24H, J = 7.2 Hz, (CH ) CH). P { H} NMR (CD CN): δ 56.4
s). Anal. Calcd for C H Ce Mn N O P : C, 39.31; H, 7.70; N,
3
2
3
EXPERIMENTAL SECTION
■
72 168
2
2
6
20 12
General Considerations. All manipulations were carried out
under nitrogen by standard Schlenk techniques. Solvents were purified
by standard procedures, distilled, and degassed prior to use. NMR
spectra were recorded on a Bruker ARX 400 spectrometer operating at
3.82. Found: C, 38.73; H, 8.23; N, 3.80.
Kinetic Study of Decomposition of 3 in CH
decomposition of 3 in CH Cl at room temperature was studied by
monitoring the absorbance of the LMCT band of the MnO
at 526 nm. The first-order rate constant, kobs, was determined
according to the equation, A = A + (A − A )exp(−kobst), where A
and A are the initial and final absorbance, respectively. Rate constants
Cl . The kinetics of
2
2
2
2
−
moiety
4
1
31
19
4
00, 161.9, and 376.4 MHz for H, P, and F, respectively. Chemical
1
shifts (δ, ppm) were reported with reference to SiMe ( H), H PO
t
∞
0
∞
0
4
3
4
31
19
(
P), and CF C H ( F). Infrared (IR) and UV−visible spectra were
∞
3
6
6
recorded on Perkin-Elmer 16 PC FT-IR and Hewlett-Packard 8452A
UV−visible spectrophotometers, respectively. Elemental analyses were
performed by Medac Ltd., Surrey, U.K.
for decomposition of 3 in CH Cl /MeCN solutions were determined
similarly.
2 2
Stoichiometric Oxidation of Ethylbenzene with 3. A solution
of 3 (36 mg, 0.0163 mmol) in ethylbenzene/MeCN (1:5, 4 mL) or
neat ethylbenzene (4 mL) was stirred at room temperature under
nitrogen for 2 h. The organic products were analyzed by GLC using
PhBr as the internal standard.
15
The ligand K[N(i-Pr PO) ] was prepared according to a literature
2
2
1
5
16
method. The compounds Ce[N(i-Pr PO) ] Cl (1), AgReO4,
2
2 3
1
7
18
AgMnO , (n-Bu N) [MoO ], and (n-Bu N)[Cp*MoO ] (Cp* =
4
4
2
4
4
3
5
19
η -C Me ) were synthesized as described elsewhere. All other
5
5
reagents were purchased from commercial sources and used as
received.
Iodometric Titration. The Mn oxidation state of the inorganic
product for the oxidation of ethylbenzene by 3 was determined by an
2
0
Synthesis of Ce[N(i-Pr PO) ] (ReO ) (2). To a solution of
iodometric method. At the end of reaction, a 1.0 mL aliquot of the
solution was transferred to a 10 mL volumetric flask containing 0.019 g
of n-Pr NI. Glacial acetic acid (1 mL) was added, followed by CH Cl ,
2
2 3
4
AgReO (35.8 mg, 0.1 mmol) in MeCN (10 mL) was added a solution
4
of 1 (101.6 mg, 0.1 mmol) in MeCN (10 mL), and the mixture
4
2
2
Table 1. Crystallographic Data and Experimental Details for Heterometallic Ce-O-M Complexes 3−6
compound
formula
3
4
5
6
C H168Ce2
C H168Ce2
C H Ce
24 56
C H87.33Ce
7
2
72
44
Mn N O P
MoN O P
N O P
Mo N O10.67P4
2
6
20 12
6
16 12
4
10
4
2
2
fw
2199.88
173(2)
0.71073
2121.94
824.73
173(2)
1271.04
T, K
173(2)
0.71073
triclinic
172.9(3)
0.71073
wavelength, Å
cryst syst
space group
a, Å
1.54178
hexagonal
P31c
monoclinic
P21
monoclinic
C2/c
̅
P1
̅
12.5734(2)
12.5734(2)
38.3973(10)
12.6951(11)
13.0938(12)
17.7937(16)
78.786(2)
79.1360(10)
62.8470(10)
2564.7(4)
1
9.6503(2)
14.9612(2)
13.6574(2)
18.7983(4)
14.1982(4)
21.6789(4)
b, Å
c, Å
α, deg
β, deg
120
108.785(2)
95.839(2)
γ, deg
V, Å3
5256.98(18)
2
1866.82(5)
2
5756.1(2)
4
Z
ρcalc, Mg/m3
μ, mm−
F(000)
no. of reflns
no. of indep reflns
Rint
1.390
1.374
1.467
11.479
852
1.467
1.368
2611
1
1.326
1.236
2296
1108
32 867
3815
8632
6880
33 384
5638
12671
4853
0.0344
1.008
0.0240
0.0507
1.017
0.0508
0.1196
0.0534
0.1206
0.0371
1.021
0.0250
0.0794
0.0261
0.0803
a
GOF
1.034
b
R1
0.0324
0.0982
0.0355
0.1009
0.0752
c
wR (I > 2σ)
0.2056
2
R1
0.0985
wR (all data)
2
0.2348
a
GOF = [∑w(|F | − |F |) /(N − Nparam)]1/2. R = (∑(|F | − |F |)/∑|F |. wR = [∑w (|F | − |F |) /∑w |F | ]1/2.
2
b
c
2
2
2
2
o
c
obs
1
o
c
o
2
o
c
o
2
557
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Inorganic Chemistry
Article
Table 2. Selected Bond Lengths (Å) and Angles (deg) for Complexes 3−6
3
(M = Mn)
4 (M = Mo)
5
6 (M = Mo)
Ce-O(P)
Ce-O(M)
M-Ot
2.2258(17)−2.631(16)
2.528(8)
2.267(6)−2.320(6)
2.236(6)
2.232(5)−2.238(5)
2.2568(16)−2.2775(17)
2.1640(17)
1.571(5)−1.576(8)
1.643(8)−1.736(7)
1.5438(18)−1.5493(17)
1.584(2)−1.588(2)
180
1.677(17)−1.697(7)
1.872(6)
1.7154(19)−1.7204(19)
1.8606(17)
M-O(Ce)
P-O
1.522(6)−1.535(8)
1.538(11)−1.585(7)
141.8(4)−150.5(4)
125.7(5)−126.9(6)
1.538(6)−1.549(6)
1.558(7)−1.605(7)
1.5407(17)−1.5479(17)
1.589(2)−1.592(2)
168.74(10)
P-N
Ce-O-M
P-N-P
128.04(13)
131.1(4)−131.3(5)
128.91(14)
Scheme 1. Synthesis of Heterometallic Cerium(IV) Complexes
3
1
1
−1
to bring the total volume to 10 mL, and the mixture was stirred
thoroughly. An aliquot of the solution was promptly transferred to a
cuvette, and the UV/vis spectrum was recorded. The concentration of
P { H} NMR (CDCl ): δ 54.4 (s). IR (KBr, cm ): 1384 (s)
3
[ν(NO )]. Anal. Calcd for C H CeN O P : C, 34.95; H, 6.84; N,
6.79. Found: C, 35.09; H, 6.96; N, 6.65.
Synthesis of Ce[N(i-Pr PO) ] (Cp*MoO ) (6). To a solution of
3
24 56
4
10 4
−
−
−1
I3 formed was calculated assuming that ε (I ) = 26 200 M
365
3
2
2 2
3 2
−1
21
cm . The result of iodometric titration indicated that the inorganic
product is about four oxidizing levels below the monomeric unit of 3.
Therefore, the Ce and Mn oxidation states of the inorganic product
are tentatively assigned as +3 and +4 (vide infra), respectively.
[
n-Bu N][Cp*MoO ] (103 mg, 0.197 mmol) in THF (5 mL) was
4 3
added a solution of 5 (81.4 mg, 0.0985 mmol) in THF (5 mL) at −78
C, and the mixture was warmed to room temperature and stirred for
h. The solvent was pumped off, and the residue was extracted with
Et O (10 mL). Recrystallization from MeCN at −18 °C afforded red
°
1
Reaction of 1 with AgBF . A mixture of 1 (10.4 mg, 0.01 mmol)
4
2
and AgBF (2 mg, 0.01 mmol) in CD CN (0.5 mL) was stirred at
1
4
3
crystals. Yield: 78 mg (63%). H NMR (CDCl ): δ 1.20 (m, 48H,
(
{
N O P : C, 41.97; H, 6.88; N, 2.22. Found: C, 41.83; H, 6.62; N,
2
1
3
room temperature for 10 min and filtered. H NMR (CD CN): δ 1.20
m, 72H, (CH ) CH), 2.01 (sept, 12H, J = 7.2 Hz, (CH ) CH).
H} NMR (CD CN): δ 56.4 (s). F { H} NMR (CD CN): δ
31
3
CH ) CH), 1.98 (m, 8H, (CH ) CH), 2.07 (s, 30H, C Me ).
P
31
3
2
3
2
5
5
(
{
P
1
3
2
3 2
H} NMR (CDCl ): δ 51.4 (s). Anal. Calcd For C H CeMo -
1
19
1
3
44 86
2
3
3
2
10 4
−
151.9 (s).
.26.
Synthesis of [Ce{N(i-Pr PO) } ] (μ-MoO ) (4). To a solution of
2
2 3 2
4
Catalytic Oxidation of Organic Sulfides. Typically, a mixture of
methyl p-tolyl sulfide (10 μL, 0.0743 mmol), tert-butyl hydroperoxide
0.11 mmol, 18.5 μL of 6.0 M solution in decane), and catalyst (0.003
mmol) in CH Cl (2 mL) was stirred at room temperature for 2 h.
[
Ce{N(i-Pr PO) } ][BF ] prepared from 1 (203 mg, 0.2 mmol) and
2 2 3 4
AgBF (39 mg, 0.2 mmol) in MeCN (5 mL) as described above was
4
(
added a solution of [n-Bu N] [MoO ] (64 mg, 0.1 mmol) in MeCN
4
2
4
2
2
(
5 mL), and the mixture was stirred at room temperature for 2 h. The
The organic products were analyzed by GLC using iodobenzene as the
internal standard.
solvent was removed under reduced pressure, and the residue was
extracted with hexane. Concentration and cooling at −10 °C afforded
1
X-ray Crystallography. Crystallographic data, experimental de-
tails, and selected bond lengths and angles for complexes 3−6 are
listed in Tables 1 and 2, respectively. Intensity data were collected on a
Bruker Smart CCD diffractometer and an Oxford Xcalibur PD X-ray
diffractometer. The collected frames were processed with the software
SAINT. Structures were solved by the direct methods and refined by
full-matrix least-squares on F using the SHELXTL software package.
Atomic positions of non-hydrogen atoms were refined with anisotropic
yellow crystals. Yield: 62 mg (53%). H NMR (CDCl ): δ 1.15−1.33
3
31
1
(
m, 72H, (CH ) CH), 1.94−2.02 (m, 12H, (CH ) CH). P { H}
NMR (CDCl ): δ 53.1 (s). We were not able to obtain satisfactory
3
2
3 2
3
analytical data. However, this compound has been well-characterized
by a spectroscopic method and X-ray diffraction.
2
2
Synthesis of trans-Ce[N(i-Pr PO) ] (NO ) (5). To a solution of
2
2 2
3 2
2
23
(
NH ) Ce(NO ) (110 mg, 0.2 mmol) in MeCN (10 mL) was added
4 2 3 6
K[N(i-Pr PO) ] (128 mg, 0.4 mmol), and the mixture was stirred at
2
2
−
room temperature for 2 h. The solvent was pumped off, and the
parameters. The MnO moiety in the complex cation of 3 is 50:50
4
residue was extracted with Et O. Slow evaporation of the solvent
disordered, and the two disordered parts are related by a 2-fold axis. In
complex 4, the Mo(1), O(8), and O(9) atoms are disordered over two
symmetry-equivalent positions with 50% occupancy.
2
1
afforded orange crystals. Yield: 90 mg (55%). H NMR (CDCl ): δ
3
1
.16 (m, 48H, (CH ) CH), 1.94 (sept, 8H, J = 7.2 Hz, (CH ) CH).
3
2
3
2
2
558
dx.doi.org/10.1021/ic302567e | Inorg. Chem. 2013, 52, 2556−2563

Inorganic Chemistry
Article
RESULTS AND DISCUSSION
assume that 3 completely ionizes to the ion pair [Ce{N(i-
■
+
−
Pr PO) } ] BF in acetonitrile solution. On the other hand,
2
2
3
4
Perrhenate Complex. The syntheses of heterometallic
Ce(IV) complexes are summarized in Scheme 1. Treatment of
complex 3 decomposes readily in less polar solvents, such as
CH Cl , as evidenced by UV/vis and NMR spectroscopy.
2
2
1
with AgReO in acetonitrile afforded the Ce(IV)/Re(VII)
4
Purple solutions of 3 in CH Cl gradually turned brown and
2
2
complex Ce[N(i-Pr PO) ] (ReO ) (2). Complex 2 is air-stable
2
2 3
4
3
1
1
the permanganate LMCT band vanished completely in ca. 2 h
at room temperature. The 31P { H} NMR spectrum of the
resulting brown solution showed a broad resonance at δ 59.6
ppm, which is close to that for Ce[N(i-Pr PO) ] (cf. δ 60
in both the solid state and solution. The P { H} NMR
1
spectrum of 2 in C D displayed a singlet at δ 52.3 ppm, which
6
6
1
5
is more downfield than that of 1 (δ 49.5 ppm). We were not
able to assign the ReO stretch for 2 due to overlap with
intense ν(P−O) bands of the ligand in the IR spectrum. A
2
2 3
15
ppm). After the brown solution was left to stand in air
24
overnight, a dark precipitate (presumably MnO ) and a pale
preliminary X-ray diffraction study (Figure 1) revealed that 2
2
1
−
yellow solution, from which Ce[N(i-Pr PO) ] could be
is a 7-coordinated compound containing an η -ReO4 ligand.
Unfortunately, the crystal structure has not been refined
satisfactorily due to disorder problems.
2 2 3
isolated, were formed. Under the same conditions, no
significant UV/vis and NMR spectral change was found for
[
n-Bu N][MnO ] in CH Cl . The kinetics of the degradation
4 4 2 2
of 3 in CH Cl has been studied by monitoring the absorbance
2
2
of the LMCT band at 526 nm (Figure 3). At 25 °C, the decay
of 3 in CH Cl followed first-order kinetics with a rate constant
2
2
−4
−1
(
kobs) of (6.0 ± 0.1) × 10 s . The decomposition of 3 in
CH Cl was found to be decelerated by addition of acetonitrile.
2
2
Thus, the kobs for decomposition of 3 in 1:4 and 1:1 (v/v)
CH Cl /MeCN mixtures were determined to be (3.8 ± 0.1) ×
2
2
−
4
−4 −1
1
0
and (2.5 ± 0.1) × 10 s , respectively. At a lower
concentration of acetonitrile (CH Cl /MeCN volume ratio >
2
2
1
), the decomposition of 3 did not follow simple first-order
kinetics and appeared to consist of more than one step.
Additional study is required to determine the rate constants of
this complex, multistep kinetics. The ease of decomposition of
3
in CH Cl can be attributed to coordination and activation of
2 2
−
the MnO4 moiety by the electrophilic Ce(IV) center. Lewis
acid induced decomposition of MnO4 is well-documented.
−
Recently, Lau and co-workers reported that, upon coordination
Figure 1. Molecular structure of 2. Isopropyl groups of the ligands are
omitted for clarity. Symmetry code: A = −y + 1, x − y + 1, z; B = −x +
y, −x + 1, z.
−
of Lewis acids, such as BF , to the MnO group, MnO
3
4
undergoes intramolecular O−O coupling to generate O and
2
26
MnO . We believe that a similar pathway is responsible for
2
the decomposition of 3 in CH Cl .
2
2
Permanganate Complex. Similarly, treatment of 1 with
Oxidation of Ethylbenzene by 3. 3 can oxidize
ethylbenzene in acetonitrile, but rather slowly. The oxidation
of ethylbenzene by 3 in acetonitrile (ethylbenzene/MeCN, 1:5
v/v) at room temperature afforded acetophenone in 15% yield
AgMnO in acetonitrile afforded a purple solid (3) analyzed as
4
“
Ce[N(i-Pr PO) ] (MnO )”. While complex 3 is stable in
2 2 3 4
acetonitrile, it decomposes readily in other organic solvents,
such as CH Cl2 (vide infra). Recrystallization of 3 from
2
(
with respect to the monomeric unit of 3, vide infra) in 2 h.
acetonitrile at −18 °C afforded purple crystals identified as
+
−
The yield increased to ca. 38% in 5 h, and no further increase
was found in a longer time. When CH Cl was used as the
[
Ce {N(i-Pr PO) } (MnO )] (MnO ) by X-ray diffraction.
2 2 2 6 4 4
2
2
Figure 2 shows the structure of the complex cation in 3. The
permanganate moiety in the complex cation is 50:50
disordered, and the two disordered parts are related by a 2-
fold axis. A weak interaction was found between the disordered
permanganate and the two Ce(IV) centers [Ce(1)−O(3) and
Ce(1A)−O(3A) distances of 2.528(8) Å, cf. 2.236(6) Å for the
molybdate analogue 4 (vide infra)]. The geometry around Ce
in 3 can be described as monocapped octahedral with Ce−
O(P) distances of 2.2258(17)−2.2631(16) Å that are
solvent (ethylbenzene/CH Cl , 1:5 v/v), acetophenone was
2
2
obtained in 10% yield in 2 h. The yield remained the same in 10
h, suggesting that the complex had decomposed. On the other
hand, acetophenone was formed almost quantitatively (vide
infra) in 2 h from the reaction of 3 with neat ethylbenzene.
Under the same conditions, very little acetophenone/benzoic
acid was detected for the reaction of [n-Bu N][MnO ] with
4
4
neat ethylbenzene in 2 h. The oxidation of neat ethylbenzene
1
5
with 3 gave a brown solution that possibly contained a Ce(III)
comparable to those in Ce[N(i-Pr PO) ] Cl. Although
2
2 3
31
species ( P resonance at δ 59.6 ppm). Iodometric titration
indicated that the inorganic product is about four oxidizing
levels below the monomeric unit of the Ce(IV)/Mn(VII)
starting material 3. After the brown solution was left to stand in
permanganate-based coordination polymers, such as [AgMn-
2
5
O ] , have been isolated, to our knowledge, 3 is the first
4
n
structurally characterized molecular heterometallic permanga-
nate complex showing a Ce−O(Mn) interaction.
The UV/vis spectrum of 3 in acetonitrile displayed the
permanganate-based ligand-to-metal charge transfer (LMCT)
band centered at 526 nm (Figure 3) that is very similar to that
air overnight, a brown precipitate (presumably MnO
with a pale yellow solution, from which crystals of Ce[N(i-
Pr PO) could be isolated, was formed. Therefore, it is
2
) along
]
2 3
2
1
31
1
of [n-Bu N][MnO ]. Both the H and the P { H} NMR
reasonable to assume that the Ce and Mn oxidation states of
the inorganic product are +3 and +4, respectively, as
represented by the following stoichiometric equation (eq 1).
4
4
spectra of 3 in CD CN are almost identical with those for
3
[
Ce{N(i-Pr PO) } ][BF ] (vide infra). Thus, it is reasonable to
2 2 3 4
2
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Inorganic Chemistry
Article
Figure 2. (a) Molecular structure the complex cation in 3. (b) A view of the disordered permanganate moiety. Isopropyl groups of the ligands are
omitted for clarity. The ellipsoids are drawn at 30% probability level. Symmetry code: A = −y + 1, x − y, z; B = −x + y + 1, −x + 1, z; C = −y + 1, −x
+
1, −z + 3/2; D = −x + y + 1, y, −z + 3/2; E = x, x − y, −z + 3/2.
2
8
[
CeIV {N(i‐Pr PO) } (MnVIIO4)](MnVIIO4) + 2PhCH2
acid. We believe that a similar mechanism was involved in the
oxidation of ethylbenzene by 3. Attempts to use 3 as a catalyst
for the oxidation of ethylbenzene with terminal oxidants, such
as tert-butyl hydroperoxide or PhIO, failed, possibly because the
reoxidation of the Ce(III)-Mn(IV) species to 3 is difficult.
Molybdate Complexes. No reaction was found between 1
2
2
2 6
III
IV
CH → 2“Ce [N(i‐Pr PO) ] Mn O ”
3
2
2 3
2
+
2PhC(O)CH + 2H O
(1)
3
2
Assuming that the oxidation of ethylbenzene to acetophe-
none is a 4-electron process and dimeric 3 is an overall 8-
electron oxidant, the yield of acetophenone was determined
to be ca. 100% based on complex 3. The oxidation of other
alkylbenzenes, such as cumene, has not been examined due to
the poor solubility of 3 in these alkylbenzenes. Lau and co-
and Ag MoO , presumably due to poor solubility of Ag MoO
in acetonitrile. Thus, an alternative route to Ce(IV)-molybdate
starting from [Ce{N(i-Pr
attempted. Treatment of 1 with AgBF
immediate formation of AgCl. The resulting cationic Ce(IV)
2
4
2
4
27
+
2−
PO)
}
]
and [MoO
]
was
2
2
3
4
in MeCN resulted in
4
workers reported that Lewis acids, such as BF , and metal
complex, possibly [Ce{N(i-Pr PO) } ][BF ], has been charac-
3
2
2
3
4
triflates accelerate the KMnO4 oxidation of hydrocarbons
through the coordination of the MnO group to the Lewis
terized by NMR spectroscopy. However, “[Ce{N(i-Pr PO) } ]-
2 2 3
[BF ]” is not very stable in solution. Recrystallization of
4
2
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Inorganic Chemistry
Article
Figure 3. UV/vis spectral change for the decomposition of 2 in CH Cl (1 mM) at 298 K (time interval = 6 min). Inset: Plot of log(A − A )/(A −
2
2
t
∞
o
A ) versus time for the LMCT band (λ = 526 nm).
∞
Figure 4. Molecular structure of 4. Isopropyl groups of the ligands are omitted for clarity. The Mo(1), O(8), and O(9) atoms are disordered over
two symmetry-equivalent positions with 50% occupancy. Only one of the disordered parts containing Mo(1), O(8), and O(9) is shown here. The
ellipsoids are drawn at 30% probability level. Symmetry code: A = −x + 2, −y + 2, −z + 1.
[
Ce{N(i-Pr PO) } ][BF ] from CH Cl /hexane resulted in the
Treatment of in situ prepared [Ce{N(i-Pr PO) } ][BF ]
2 2 3 4
2
2
3
4
2
2
Ce(IV/III) reduction and formation of Ce[N(i-Pr PO) ] .
with [n-Bu
N][Cp*MoO ] in acetonitrile afforded a new
4 3
2
2 3
Therefore, [Ce{N(i-Pr PO) } ][BF ] was prepared in situ from
species, which is tentatively formulated as a monomolybdate
2
2
3
4
1
complex, Ce[N(i-Pr PO) ] (Cp*MoO ), on the basis of H
1
and AgBF in acetonitrile and used for the subsequent
2
2 3
3
4
NMR data (ratio of the integration of the Cp* to [N(i-
reactions. Treatment of as-prepared [Ce{N(i-Pr PO) } ][BF ]
2
2
3
4
−
Pr PO) ] signals ∼ 1:3). This monomolybdate species is not
with 0.5 equiv of [n-Bu N] (MoO ) afforded the trinuclear
2
2
4
2
4
31
1
stable in solution, as evidenced by NMR spectroscopy, and
gradually rearranged to give the bis-molybdate complex
Ce[N(i-Pr PO) ] (Cp*MoO ) (6), which has been charac-
complex [Ce{N(i-Pr PO) } ] (μ-MoO ) (4). The P { H}
2
2
3 2
4
NMR spectrum in CDCl displayed a singlet at δ 53.1 ppm.
3
2
2 2
3 2
The solid-state structure of 4 (Figure 4) consists of two
terized by X-ray diffraction. Alternatively, 6 could be
synthesized in higher yield by reaction of a Ce(IV) bis(chelate)
starting material and [n-Bu N][Cp*MoO ]. The dinitrate
+
symmetry-related [Ce{N(i-Pr PO) } ] fragments linked by a
2
2 3
μ2-O,O’-MoO42 anion. An inversion center was found at the
center of the molecule. The Mo, O8, and O9 atoms of the
molybdate are 50:50 disordered. The Ce−O(P) distances in 4
2.267(6)−2.320(6) Å] are similar to those in 3. The Ce−
O(Mo) distances [2.236(6) Å] are quite short and comparable
−
4
3
complex trans-Ce[N(i-Pr PO) ] (NO ) (5) has been prepared
2
2 2
3 2
from [NH ] [Ce(NO ) ] and 2 equiv of K[N(i-Pr PO) ]. X-
4
2
3 6
2
2
[
ray crystallography confirmed that the two nitrates in 5 are
2
mutually trans and bind to the Ce(IV) center in an η -fashion
−
to those in H (CeL ) Mo O , where L
= [CpCo{P(O)-
4
OEt 6
9
38
OEt
(Figure 5). The Ce−O(nitrate) distances in 6 [2.458(6)−
−
10
29
(
OEt) } ] [2.217(7)−2.407(8) Å], and the Ce−O−Mo
2
3
2.516(7) Å] are similar to those in (L ) Ce(NO ) .
OEt 2
3 2
angle is 141.8(4) and 150.5(4)°. The bridging Mo−O(Ce)
bond [1.872(6) Å] is obviously longer than the terminal Mo−
O bonds [1.677(17)−1.697(7) Å].
Treatment of 5 with 2 equiv of [n-Bu N][Cp*MoO ] afforded
4 3
1
6. H NMR spectroscopy confirmed that the ratio of the Cp*
31
1
and imidodiphosphinate ligands in 6 is 1:1. The P { H} NMR
2
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Inorganic Chemistry
Article
Table 3. Catalytic Oxidation of Organic Sulfides with tert-
a
Butyl Hydroperoxide (TBHP)
catalyst
R
% yield
I:II
4
4
6
H
99
99
0
24:1
24:1
Me
H
Ce[N(i-Pr O) ] Cl
Me
H
0
2
2 3
[
[
n-Bu N] (MoO )
75
84
4.3:1
3:1
4
2
4
n-Bu N] (MoO )
Me
4
2
4
a
Experimental conditions: Sulfide (0.0743 mmol), TBHP (0.1114
mmol), catalyst (0.003 mmol, 4 mol %), stirred in CH Cl at RT for 2
h. Yields based on sulfide determined by GLC.
2
2
Figure 5. Molecular structure of 5. Isopropyl groups of the ligands are
omitted for clarity. The ellipsoids are drawn at 30% probability level.
spectrum showed a singlet at δ 51.4 ppm. The molecular
structure of 6 is shown in Figure 6. Unlike 3 and 4, the
31
complexes, 6 cannot catalyze the sulfoxidation. Ce(IV)-LOEt
complexes, such as Ce[N(i-Pr PO) ]Cl, are not active catalysts,
2
3
suggesting that the Mo rather than Ce is the catalytic site for
the Ce-Mo-catalyzed oxidation. Recent studies suggest that the
mechanism of the Mo-catalyzed sulfoxidation with TBHP
involves the formation of a Mo alkylperoxo/peroxo inter-
mediate and the nucleophilic attack of a peroxo oxygen atom by
32
the sulfide substrate. We believe that a similar pathway is
involved in 4-catalyzed sulfoxidation. The Ce/Mo complex 4 is
a more selective catalyst than [n-Bu N][MoO ], suggesting that
4
4
+
the [Ce{N(i-Pr PO) } ] fragments in 4 have an influence on
2
2 3
the selectivity and reactivity of the Mo alkylperoxo/peroxo
intermediates. Currently, an effort is being made to isolate and
characterize the Ce-Mo alkylperoxo/peroxo species and to
explore their reactivity.
CONCLUSIONS
■
Figure 6. Molecular structure of 6. Isopropyl groups of the ligands are
omitted for clarity. The ellipsoids are drawn at 30% probability level.
Symmetry code: A = −x, y, 1/2 − z.
We have synthesized and structurally characterized hetero-
metallic Ce(IV) permanganate and molybdate complexes
supported by the imidodiphosphinate ligand [N(i-Pr PO) ]
−
2
2
featuring Ce(IV)-O-M linkages. The Ce(IV) permanganate
complex 3 decomposes readily in CH Cl and can oxidize neat
ethylbenzene efficiently at room temperature. These results are
geometry around the Ce atom in 6 is pseudo-octahedral. The
Ce−O−Mo angles [168.74(10)°] are larger than those in 4,
and the Ce−O(Mo) bond distance [2.1640(17) Å] is shorter
than that in 4 [2.236(6) Å], suggesting that the Ce−OMo
interactions in 6 are quite strong and this probably explains why
monomolybdate species rearranged to 6 in solution easily. The
bridging Mo−O(Ce) bond [1.8606(17) Å] is longer than the
terminal ones [1.7154(19)−1.7204(19) Å], which are longer
2
2
suggestive of activation of permanganate by the electrophilic
Ce(IV) center. The Ce(IV) molybdate complex 4 is a more
−
selective sulfoxidation catalyst than free [MoO
] , indicating
4
that the selectivity and reactivity of the Mo-bound alkylperoxo
+
active species can be influenced by the [Ce{N(i-Pr
PO) } ]
2
2 3
fragments.
−
19b
than those in [Cp*MoO ] [1.742(4)−1.765(4) Å].
3
A preliminary study showed that the Ce(IV) molybdate
complex 4 is a catalyst for oxidation of organic sulfides (Table
ASSOCIATED CONTENT
■
*
S
Supporting Information
3
). For example, the oxidation of phenyl methyl sulfide with
tert-butyl hydroperoxide (TBHP) in the presence of 4 mol % of
afforded a mixture of sulfoxide (I) and sulfone (II) in 99%
CIF file. This material is available free of charge via the Internet
at http://pubs.acs.org.
4
yield and a selectivity of 24:1. The sulfoxide was formed
selectively, indicating that an electrophilic alkylperoxo/peroxo
AUTHOR INFORMATION
■
*
30
active species is responsible for the oxo transfer. Similar yield
and selectivity were found for p-tolyl methyl sulfide. Under the
same conditions, [n-Bu N][MoO ] can also catalyze the
Corresponding Author
E-mail: chleung@ust.hk.
4
4
oxidation of the sulfides, but with lower selectivity (I:II ratio
Notes
of 4.3 and 3, respectively). Unlike other Mo cyclopentadienyl
The authors declare no competing financial interest.
2
562
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Inorganic Chemistry
Article
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ACKNOWLEDGMENTS
■
The support from the Hong Kong Research Grants Council
and the Hong Kong University of Science and Technology
projects 602310 and RPC11SC18) is gratefully acknowledged.
1
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