Heterobimetallic cerium(IV) oxo clusters supported by a tripodal oxygen ligand

Article abstract of DOI:10.1039/c6dt00678g

Heterometallic Ce^IV/M (M = Mo^VI, Re^VII, V^V) oxo clusters supported by the Kl?ui tripodal oxygen ligand [(η⁵-C₅H₅)Co{P(O)(OEt)₂}₃]^- (L_OEt^-) have been synthesized and structurally characterized, and the catalytic activity of the Ce^IV/V^V oxo cluster in the oxidation of thioanisoles has been studied. Treatment of [Ce(L_OEt)Cl₃] (1) with [Ag₂MoO₄] afforded the reported Ce^IV/Mo^VI cluster [H₄(CeL_OEt)₆Mo₉O₃₈] (2), whereas that with [AgReO₄] yielded the Ce^IV/Re^VII cluster [{L_OEtCe(ReO₄)₂(H₂O)(μ-ReO₄)}₂] (3) that contains an 8-membered Ce₂Re₂O₄ ring. Treatment of 1 with [Ag₃VO₄] afforded the Ce^IV/V^V cluster [H₂(CeL_OEt)₄(VO)₄(μ₄-O)(μ₃-O)₁₂] (4) containing a {Ce₄V₄O₁₃} oxo-metallic core. The solid-state structure of 4 consists of four {VO₄}^3- units bridged by four {L_OEtCe³⁺} moieties and a μ₄-oxo ligand. Each Ce atom in 4 is 9-coordinated, whereas the geometry around each V atom is pseudo square pyramidal with a terminal oxo at the apical position. Cluster 4 is an active catalyst for the oxidation of substituted thioanisoles with tert-butyl hydroperoxide. For example, the oxidation of thioanisole with tert-butyl hydroperoxide in the presence of 0.01 mol% of 4 gave a ca. 30 : 1 mixture of the sulfoxide and sulfone products in 96% yield.

Full text of DOI:10.1039/c6dt00678g

Dalton
Transactions
View Article Online
View Journal
PAPER
Heterobimetallic cerium(IV) oxo clusters supported
by a tripodal oxygen ligand†
Cite this: DOI: 10.1039/c6dt00678g
Kang-Long Wong,‡ Yat-Ming So,‡ Guo-Cang Wang, Herman H.-Y. Sung,
Ian D. Williams* and Wa-Hung Leung*
Heterometallic CeIV/M (M = MoVI, ReVII, VV) oxo clusters supported by the Kläui tripodal oxygen ligand
[(η5-C5H5)Co{P(O)(OEt)2}3]− (LOEt−) have been synthesized and structurally characterized, and the catalytic
activity of the CeIV/VV oxo cluster in the oxidation of thioanisoles has been studied. Treatment of [Ce(LOEt
)
Cl3] (1) with [Ag2MoO4] afforded the reported CeIV/MoVI cluster [H4(CeLOEt)6Mo9O38] (2), whereas that
with [AgReO4] yielded the CeIV/ReVII cluster [{LOEtCe(ReO4)2(H2O)(μ-ReO4)}2] (3) that contains an 8-mem-
bered Ce2Re2O4 ring. Treatment of 1 with [Ag3VO4] afforded the CeIV/VV cluster [H2(CeLOEt)4(VvO)4(μ4-
O)(μ3-O)12] (4) containing a {Ce4V4O13} oxo-metallic core. The solid-state structure of 4 consists of four
{VO4}3− units bridged by four {LOEtCe3+} moieties and a μ4-oxo ligand. Each Ce atom in 4 is 9-co-
ordinated, whereas the geometry around each V atom is pseudo square pyramidal with a terminal oxo at
the apical position. Cluster 4 is an active catalyst for the oxidation of substituted thioanisoles with tert-
butyl hydroperoxide. For example, the oxidation of thioanisole with tert-butyl hydroperoxide in the pres-
ence of 0.01 mol% of 4 gave a ca. 30 : 1 mixture of the sulfoxide and sulfone products in 96% yield.
Received 20th February 2016,
Accepted 26th April 2016
DOI: 10.1039/c6dt00678g
www.rsc.org/dalton
serve as structural and functional models of vanadia/ceria cata-
lysts have not been reported.
Introduction
Ceria-containing materials have attracted much attention
Heterometallic CeIV/M oxo complexes and clusters are
because of their widespread applications in heterogeneous cat- generally synthesized by (a) self-assembly of CeIV compounds
alysis.1 Owing to their redox properties, ceria-based materials with metal salts in aqueous media or (b) hydrolysis of CeIV
and nanoparticles have been used as supports for various tran- alkoxides in the presence of a metal salt.5 Also, CeIV-containing
sition-metal-catalyzed reactions, including NOx reduction, oxi- polyoxometalates have been prepared by complexation of CeIV
dative dehydrogenation, alkene epoxidation, and methanation with unsaturated polyoxometalate precursors.6 Nevertheless,
of CO2.2 It is believed that the catalytic activity of ceria-contain- very few molecular CeIV/VV clusters containing Ce–O–V
ing catalysts is dependent upon the facile CeIV–CeIII redox linkages have been reported.7 Recently, a CeIII/VIV,V poly-
couple and the ability of ceria in storage, release, and transport oxovanadate, (n-Bu4N)2[{CeIII(dmso)3}2VIVVV11O33Cl] (dmso = di-
of oxygen, as well as the metal–ceria interactions. Of special methylsulfoxide), has been synthesized and used as a catalyst
interest are vanadium oxides supported on ceria that exhibit for the photo-degradation of indigo.7c,d
very high catalytic activity in oxidative dehydrogenation reac-
Owing to its π-donating capability, the Kläui tripodal ligand
−
tions.3 To better understand the structure and reactivity of [Co(η5-C5H5){P(O)(OEt)2}3]− (denoted as LOEt
hereafter,
vanadia/ceria catalysts, some model systems based on vanadia Scheme 1) has proven to be an excellent supporting ligand for
monomers and clusters on the CeO2 (111) surface have been tetravalent metal ions such as Ti4+, Zr4+, and Ce4+.8 In acidic
investigated.4 Nevertheless, the exact mechanisms of vanadia/ solutions, MIVLOEt (M = Ti, Zr, Ce) complexes can undergo
ceria-based catalytic reactions are not well understood at the
atomic level. Molecular Ce/V complexes and clusters that can
Department of Chemistry, The Hong Kong University of Science and Technology,
Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China.
E-mail: chwill@ust.hk, chleung@ust.hk
†Electronic supplementary information (ESI) available. CCDC 1006652 and
1451006. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c6dt00678g
−
‡These authors contributed equally to this work.
Scheme 1 Structure of the Kläui tripodal ligand LOEt
.
This journal is © The Royal Society of Chemistry 2016
Dalton Trans.

View Article Online
Paper
Dalton Transactions
self-assembly with oxo anions to give oxo/hydroxo clusters,9 [Ce(LOEt)2(NO3)2] with (NH4)6[Mo7O24] in water.9c However, in
such as [LOEtZr(PO4)]4 and [H4(CeLOEt)6Mo9O38]. In an effort to the previous synthetic method, in addition of 2, a side-
better understand the influence of the electrophilic CeIV center product, [MoLOEt(O)2]2(μ-O), was produced. An attempt has
on the reactivity of metal–oxo complexes, we sought to syn- been made to prepare an analogous Ce/W oxo cluster by react-
thesize heterometallic CeIV vanadate complexes and explore ing 1 with Ag2WO4 in MeCN. Unfortunately, this reaction led
their catalytic activity.
to formation of a yellow oily species that did not crystallize.
The treatment of with equivalents of [AgReO4]
In recent work, we found that the chloride ligands in
1
3
[CeIV(LOEt)Cl3] (1) are labile and can be easily abstracted by AgI com- resulted in the formation of an octanuclear CeIV/ReVII cluster
pounds.10 For example, the reaction for 1 with Ag2O afforded a [{LOEtCe(ReO4)2(H2O)(μ-ReO4)}2] (3). Cluster 3 is soluble in Et2O,
tetranuclear CeIV oxo cluster, [H2(LOEtCe)4O7], presumably via CH2Cl2 and MeCN, but insoluble in hexanes. It is diamagnetic
the self-assembly of a mononuclear LOEtCeIV oxo/hydroxo inter- (Evans method, CDCl3 solution) and exhibited well-resolved 1H
mediate. This result led us to synthesize heterometallic CeIV and 31P {1H} NMR spectra, consistent with the CeIV/ReVII for-
oxo clusters by metathesis of 1 with AgI salts of metal–oxo mulation. The IR spectrum of 3 showed a band at 916 cm−1
−
anions such as MoO42−, ReO4 and VO43−. Herein, we report that was not observed in 1 or 2. This band is tentatively
the synthesis and structure of heterometallic CeIV/ReVII and assigned as ν(Re–O). The solid-state structure of 3 consisting
CeIV/VV oxo clusters derived from 1. The catalytic activity of the of two symmetry-related [Ce(LOEt)(ReO4)2(H2O)]+ fragments
CeIV/VV oxo cluster [H2(CeLOEt)4(VvO)4(μ4-O)(μ3-O)12] (4) that linked by two μ-O,O′-ReO4− ligands is shown in Fig. 1. Selected
features a unique {Ce4V4O13} oxo-metallic core in the oxidation bond lengths and angles are collected in Table 1. The geome-
of thioanisoles with tert-butyl hydroperoxide (TBHP) has been try around Ce is pseudo square anti-prismatic. The Ce–O(LOEt
)
examined.
distances [2.217(10)–2.246(11) Å] in 3 are comparable to those
in [Ce(LOEt)(NO3)3] [2.220(2)–2.232(2) Å] and [Ce(LOEt)Cl3]
Results and discussion
Heterometallic CeIV oxo clusters were synthesized by meta-
thesis reactions of [LOEtCeCl3] (1) with AgI compounds
(Scheme 2).
CeIV/MoVI and CeIV/ReVII clusters
The treatment of 1 with [Ag2MoO4] in MeCN gave the reported
CeIV/MoVI oxo cluster [H4(CeLOEt)6Mo9O38] that was character-
ized by NMR spectroscopy and single-crystal X-ray diffraction.
Cluster
2 was previously prepared by self-assembly of
Fig. 1 Molecular structure of 3. Thermal ellipsoids are drawn at a 30%
−
probability level. Hydrogen atoms and ethoxy groups in LOEt are
omitted for clarity.
Table 1 Selected bond lengths (Å) and angles (°) for 3
Ce(1)–O(7)
Ce(1)–O(8)
Ce(1)–O(9)
Ce(1)–O(10)
Ce(1)–O(21)
Ce(1)–O(25)
Ce(1)–O(29)
Ce(1)–O(37)
Re(1)–O(21)
Re(1)–O(22)
Re(1)–O(23)
Re(1)–O(24)
Re(2)–O(25)
Re(2)–O(26)
2.217(10)
2.246(11)
2.234(12)
2.404(12)
2.456(12)
2.498(10)
2.298(11)
2.299(12)
1.735(12)
1.739(10)
1.709(11)
1.683(14)
1.727(10)
1.744(12)
Ce(2)–O(17)
Ce(2)–O(18)
Ce(2)–O(19)
Ce(2)–O(20)
Ce(2)–O(22)
Ce(2)–O(26)
Ce(2)–O(33)
Ce(2)–O(41)
Re(4)–O(33)
Re(5)–O(37)
Re(6)–O(41)
Re(2)–O(27)
Re(2)–O(28)
Re(3)–O(29)
2.236(10)
2.220(10)
2.229(11)
2.392(12)
2.513(10)
2.431(12)
2.338(11)
2.301(11)
1.759(11)
1.739(12)
1.755(11)
1.709(19)
1.689(14)
1.767(12)
O(21)–Re(1)–O(22)
O(26)–Ce(2)–O(22)
Re(1)–O(21)–Ce(1)
Re(1)–O(22)–Ce(2)
Re(6)–O(41)–Ce(2)
108.6(5)
67.7(4)
159.2(7)
149.5(6)
148.4(6)
O(25)–Re(2)–O(26)
O(21)–Ce(1)–O(25)
Re(2)–O(25)–Ce(1)
Re(2)–O(26)–Ce(2)
Re(3)–O(29)–Ce(1)
111.9(5)
67.3(4)
158.2(6)
164.8(7)
146.9(7)
Scheme 2 Syntheses of heterometallic CeIV oxo clusters.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2016

View Article Online
Dalton Transactions
Paper
[2.226(3)–2.244(3) Å], but shorter than those in the tetranuclear
oxo cluster [H2(CeLOEt)4O7] [2.383(3)–2.457(4) Å].10 The Ce–OH2
distances [2.392(12) and 2.404(12) Å] are shorter than that in
[(LOEt)2CevO(H2O)]·MeC(O)NH2 [2.572(3) Å].11 The Ce–O(Re)
distances for the terminal perrhenate ligands [2.298(11)–2.338(11)
Å] are obviously shorter than those of the bridged perrhe-
nates [2.431(12)–2.513(10) Å]. The Ce–O–Re angles in the
Ce2Re2O4 ring are in the range of 149.5(6) to 164.8(7)°, whereas
−
those for the terminal ReO4 ligand range from 146.9(7)
to 152.7(8)°. The Re–O(terminal) bond distances [1.682(17)–
1.716(14) Å] are shorter than the bridging Re–O(Ce) bonds
[1.727(10)–1.767(12) Å].
CeIV/VV cluster
The success in the isolation of clusters 2 and 3 prompted us to
synthesize a Ce/V oxo cluster starting from 1 and [Ag3VO4]. The
treatment of 1 with [Ag3VO4] in MeCN resulted in the precipi-
tation of AgCl and a yellow solution. Recrystallization from
hexanes afforded yellow crystals that were characterized as the
CeIV/VV oxo cluster [H2(CeLOEt)4(VvO)4(μ4-O)(μ3-O)12] (4).
Cluster 4 is soluble in common organic solvents including
hexanes. It is diamagnetic (Evans method, CDCl3 solution)
and showed sharp, well-resolved NMR signals. The 31P {1H}
NMR spectrum displayed a singlet at δ 115.4 ppm that is
typical for diamagnetic CeIV–LOEt complexes.10,12 The 51V NMR
spectrum showed a signal at δ −553.3 ppm, which is within
the range reported for vanadate(V) complexes.13 We were not
able to assign the V-oxo stretch that overlaps with intense ν(P–
Fig. 2 Structure of the oxometallic {Ce4V4O13} core in 4 (top) and
coordination polyhedral representation of
4 (bottom), color code:
green, Ce; red, oxygen; blue, {VO4} tetrahedra. Selected bond lengths
(Å) and angles (°): Ce–O(LOEt) 2.373(4)–2.414(4), Ce–O(V) 2.306(3)–
2.473(4), Ce–μ4-O 2.503(4)–2.513(4), V–μ3-O 1.817(4)–1.915(4), V–Ot
1.600(4)–1.606(4); Ce–O–V 98.64(15)–102.82(14), Ce(1)–O(10)–Ce(1A)
95.50(2), Ce(1A)–O(10)–Ce(2A) 94.64(10), Ce(1A)–O(10)–Ce(2) 147.03(11),
Ce(1)–O(10)–Ce(2A) 147.03(11), Ce(1)–O(10)–Ce(2) 94.64(10), Ce(2)–O(10)–
Ce(2A) 93.68(19).
−
O) bands of the LOEt ligand in the IR spectrum. No redox
events were found in the cyclic voltammogram of 4 in MeCN
in the potential range −2.0–+1.5 V vs. Fc+/0 (Fc = ferrocene).
The solid-state structure of 4 is depicted in Fig. 2. The
asymmetric unit contains a {Ce4V4O13} oxometallic core that
exhibits an approximate C2V symmetry. The cluster core con- fore, two protons are required in order to balance the negative
sists of four Ce and four V centers that are connected by twelve charges of the dianionic [CeI4V(LOEt)4V4VO17]2− moiety. The
μ3-oxo ligands. In addition, a μ4-oxo ligand connecting the MALDI mass spectrum showed a peak at 3177.76 that can be
four Ce centers is found at the center of the cage. The average assigned to [H2(CeLOEt)4V4O17]. Although the hydrogens could
Ce–Ce, Ce–V, and V–V distances are 3.667, 3.2801, and not be unambiguously located by X-ray diffraction, we believe
2.9292 Å, respectively, indicating the absence of direct metal– that similar to [H4(CeLOEt)6Mo9O38]9c and [H2(CeLOEt)4O7],10
metal interactions. The geometry around each V center is the two protons in 4 are associated with the bridged oxo
pseudo square pyramidal, with the apical VvO group pointing positions in the {Ce4V4O13
} oxo-metallic core. Like
away from the center of the cluster. The Ce–O(LOEt) distances [H4(CeLOEt)6Mo9O38] and [H2(CeLOEt)4O7], the two hydrogens
[2.373(4)–2.414(4) Å] are comparable to those in the tetra- in 4 are non-exchangeable; no reactions were found between 4
nuclear CeIV oxo cluster [H2(CeLOEt)4O7] [2.383(3)–2.457(4) Å]10 and bases such as pyridine and Et3N.
but longer than those in [H4(CeLOEt)6Mo9O38] [2.297(7)–2.378(7)
Bond valence sum (BVS) calculations on 4 using the Valist
Å].9c The Ce–O(V) distances [2.306(3)–2.473(4) Å] are shorter program14 supported the maximal oxidation states of the
than those between Ce and the μ4-oxo ligand [2.503(4)–2.513(4) metal centres and gave the following values Ce(1) +3.84, Ce(2)
Å]. Each V is coordinated to four μ3-oxo ligands and a terminal +3.87 and V(1) +5.01, V(2) +4.96. The corresponding BVS for
oxo ligand with V–O distances of 1.817(4)–1.915(4) and 1.600(4)– the core µ4-O was just −1.33, indicating the bonds are longer
1.606(4) Å, respectively. The Ce–O–V angles are in the range than the ideal but this site is not physically possible for the H
of 98.64(15)–102.82(14)°, whereas the Ce–μ4–O–Ce angles are atoms. The µ3-O bridging two V and one Ce had BVS of −1.95
in the range of 94.64(10)–147.03(11)°.
and −1.91, the terminal VvO oxygens were −1.70 and −1.73
Since 4 is diamagnetic, the Ce and V oxidation states are and the µ3-O two Ce and one V ranged between −1.72 and
assigned to be +4 and +5, respectively. The +4 and +5 states of −1.88. These latter are the most likely locations for the two
Ce and V were further confirmed by an XPS study (see ESI†) protons, which might incipiently hydrogen bond between
and bond valence sum calculations (see later section). There- pairs of µ3-O which have O⋯O separations of 2.67–2.73 Å.
This journal is © The Royal Society of Chemistry 2016
Dalton Trans.

View Article Online
Paper
Dalton Transactions
Since the two protons would be distributed over eight such cluster 4 appears to be a promising catalyst candidate for
oxygens, then it is reasonable that no significant electron large-scale sulfoxidation given the low catalyst loading (e.g.,
density peaks are found and that the BVS are not markedly 0.001 mol%) required and high TONs found. The high activity
lower than those of the other four µ3-O oxygens. Like of catalyst 4 may be due to the fact that the V and Ce centers
H4(CeLOEt)6Mo9O38]9c and [H2(CeLOEt)4O7],10 the OH resonance are tightly bound in oxo-metallic cluster framework, rendering
1
was not observed in the H NMR spectrum of 4 (CDCl3, room the catalyst degradation difficult. Also, the electron-deficient
temperature).
CeIV centers can increase the electrophilicity of the VV centers,
thus facilitating the activation of TBHP.
Catalytic oxidation of thioanisoles
In previous studies, it was proposed that the active inter-
mediates for the V-catalyzed sulfoxidation with TBHP are V
alkylperoxo/peroxo species.16 Nucleophilic attack of the peroxo
oxygen of the alkylperoxo/peroxo species by the sulfide sub-
strate affords the sulfoxide product. We believe a similar
pathway involving a V-alkylperoxo active species is responsible
for the 4-catalyzed sulfoxidation (Scheme 3). The MALDI mass
spectrum of the reaction mixture after the 4-catalyzed sulf-
oxidation showed the signal due to the cluster (at ca. m/z 3177,
see ESI†), suggesting that the oxo-metallic core of 4 remained
intact during the catalysis. Attempts to trap/characterize the
putative V alkylperoxo/peroxo active species generated by react-
Monomeric vanadia species on the CeO2(111) surface bearing
a terminal VvO group and Ce–O–V bridges have been used to
model the active sites of vanadia/ceria catalysts.4a Since
similar structural features (namely, terminal VvO groups and
V–O–Ce bridges) are found in the Ce/V cluster 4, it is of interest
to evaluate the activity of 4 in catalytic oxidation. We found
that cluster 4 is an active catalyst for the oxidation of thio-
anisoles with tert-butyl hydroperoxide (TBHP), and the results
are summarized in Table 2. For example, the oxidation of p-tolyl
methylsulfide with TBHP at room temperature in the presence
of 0.01 mol% of 4 afforded a ca. 37 : 1 mixture of the sulfoxide
and sulfone products with a total yield of 93% in 4 h (turnover
number, TON = 9310) (entry 4). Under the same conditions,
the Ce oxo cluster [H2(CeLOEt)4O7] and the Ce/Mo and Ce/Re
clusters 2 and 3 are very poor catalysts for the oxidation,
suggesting that instead of Ce the V sites in 4 are responsible
for the sulfide oxidation. The effect of the para substituent (X
= H, Cl, Br and OMe; entries 5–8) of thioanisole on the yield of
the sulfoxide has been studied. However, no clear trend of the
para substituent effect was observed, although higher sulfox-
ide selectivity was found for the electron-rich thioanisoles (X =
Me, H). Lowering the loading of catalyst 4 did not lead to drop
in the yield and selectivity of the sulfoxidation. For example,
the oxidation of p-tolyl methyl sulfide with 0.001 mol% of 4 in
20 h resulted in the formation of the oxidized products in a
total yield of 97% (TON = 96 600) and selectivity of 31 : 1 (entry
9). Although V-catalyzed sulfoxidation is well documented,15
Scheme 3 A possible pathway for the catalytic sulfoxidation with TBHP
at a V center in cluster 4 (Ar = aryl).
Table 2 Catalytic oxidation of substituted thioanisoles with TBHPa
Catalyst loading
(mol%)
Time
(h)
Yieldb (%)
(I : II)
Total
TON
TOF
Entry
Catalyst
X
(h−1
)
1
2
3
4
5
6
7
8
9
[H2(LOEtCe)4O7]
1
1
1
Me
Me
Me
Me
H
Cl
Br
16
16
16
4
4
4
4
2
20
6 (10 : 1)
5 (10 : 1)
5 (10 : 1)
93 (37 : 1)
96 (28 : 1)
89 (15 : 1)
82 (7 : 1)
88 (26 : 1)
97 (31 : 1)
N/A
N/A
N/A
9310
9600
8920
8170
8830
96 600
N/A
N/A
N/A
2330
2400
2230
2040
4420
4830
2
3
4
4
4
4
4
4
0.01
0.01
0.01
0.01
0.01
0.001
OMe
Me
a Reaction conditions: catalyst (0.016 μmol), thioanisole (0.16 mmol), TBHP (29 µL, 5.5 M in decane, 0.16 mmol) in CH2Cl2 (3 mL), stirred at
20 °C. b Product yield was determined by GLC using internal standard method.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2016

View Article Online
Dalton Transactions
Paper
ing 4 with a stoichiometric amount of TBHP at 0 °C were reported compound [H4(CeLOEt)6Mo9O38] (2)9c by single-crystal
unsuccessful. Reaction of 4 with TBHP led to broadening of X-ray diffraction. Yield: 54 mg (60%).
51V NMR signal at δ −553.3 ppm and drop in peak intensity.
Synthesis of [{LOEtCe(ReO4)2(H2O)(μ-ReO4)}2] (3). To a solu-
However, we were not able to observe any new high-field signal(s) tion of AgReO4 (61 mg, 0.17 mmol) in MeCN (5 mL) was added
attributable to the V alkylperoxo/peroxo species,15 possibly a solution of 1 (44 mg, 0.056 mmol) in MeCN (5 mL) at
because of its low concentration. No significant change −10 °C, and the reaction mixture was stirred for 1 h at room
−
was found for the 31P NMR signal of the LOEt ligand temperature. The AgCl formed was filtered off. Concentration
(δP 115.5 ppm), indicating that the reduction of CeIV did not (to ca. 4 mL) and cooling the red filtrate at −18 °C afforded
occur. Additional work is required to elucidate the exact mech- dark red crystals that were suitable for X-ray diffraction. Yield:
anism of the 4-catalyzed sulfoxidation.
32.5 mg (40%). 1H NMR (CDCl3): δ 1.29 (t, 36H, CH3), 4.18 (m,
24H, CH2), 5.06 (s, 10H, Cp). 31P {1H} NMR (CDCl3): δ 124.25 (s).
IR (KBr, cm−1): 916 [ν(Re–O)]. Anal. Calcd for C34H74Ce2-
Co2O44P6Re6: C, 14.14; H, 2.58; Found: C, 13.28; H, 2.43.
Conclusions
Synthesis of [H2(LOEtCe)4(VvO)4(μ3-O)12(μ4-O)] (4).
A
mixture of 1 (20 mg, 0.026 mmol) and [Ag3VO4] (11 mg,
0.026 mmol) in MeCN (10 mL) was stirred at room temperature
overnight. The AgCl formed was filtered off. The solvent was
removed in vacuo, and the residue was extracted with hexanes.
Slow evaporation of the filtrate at room temperature afforded
In summary, we have developed a convenient synthetic route
to heterometallic CeIV oxo clusters starting from the CeIV chlor-
ide compound [Ce(LOEt)Cl3] (1) and AgI salts of metal–oxo
anions. The metathesis reaction of 1 with [AgReO4] yielded
[{LOEtCe(ReO4)2(H2O)(μ-ReO4)}2] (3), whereas that with [Ag3VO4]
gave [H2(CeLOEt)4(VvO)4(μ4-O)(μ3-O)12] (4) containing a unique
{Ce4V4O13} oxo-metallic core. Cluster 4 is a rare example of a
heterometallic CeIV/VV cluster that features Ce–O–V linkages
and terminal VvO groups. 4 proved to be an efficient, robust
catalyst for the oxidation of thioanisoles with TBHP. TON as
high as 96 600 has been achieved for the oxidation of p-tolyl
methylsulfide with TBHP and 0.001 mol% of 4. The synthesis
of other heterometallic CeIV–O–M (e.g. M = Ru) complexes that
can find applications in organic oxidations is underway.
1
X-ray quality crystals. Yield: 14 mg (70%). H NMR (400 MHz,
CDCl3, 25 °C): δ 1.17–1.33 (m, 72H, CH3), 4.13–4.33 (m, 48H,
CH2), 4.99–5.07 (m, 10H, Cp). 31P {1H} NMR (162 MHz, CDCl3,
25 °C): δ 115.5 (s). 51V {1H} NMR (105 MHz, CDCl3, 25 °C): δ
−553.3 (s). MALDI-MS: m/z 3177.76 [M]+. Anal. Calcd for
C68H142Ce4Co4O53P12V4: C, 25.69; H, 4.50; Found: C, 26.19; H,
4.30.
Catalytic oxidation of substituted thioanisoles with 4. Typi-
cally, a mixture of 4 (0.05 mg, 16 nmol), substituted thio-
anisole (0.16 mmol) and TBHP (29 µL, 5.5 M in decane,
0.16 mmol) in CH2Cl2 (3 mL) was stirred at 20 °C for 2–4 h.
Bromobenzene (internal standard) (3 µL) was added to the
solution, and the product yield was determined by GLC.
Experimental
General considerations
X-ray crystallography
All manipulations were carried out under nitrogen by standard Complexes 3 and 4 have been characterised by X-ray diffraction
Schlenk techniques. Solvents were purified by standard pro- (Table 3). The diffraction intensity data of 3 was collected with
cedures and distilled prior to use. NMR spectra were recorded a Rigaku Gemini™ S Ultra X-ray diffractometer with mono-
on a Bruker ARX 400 spectrometer operating at 400, 161.9, chromatized Cu-Kα radiation (λ = 1.54178 Å) at 173 K. The
100, 376.4 and 105.2 MHz for 1H, 31P, and 51V respectively. diffraction intensity data of 4 was collected with a Rigaku
Chemical shifts (δ, ppm) were reported with reference to SiMe4 SuperNova Atlas X-ray diffractometer with monochromatized
(1H), H3PO4 (31P), and VOCl3 (51V), respectively. Infrared Cu-Kα radiation (λ = 1.54178 Å) at 100 K. Diffraction data of 3
spectra were recorded on a Perkin-Elmer 16 PC FT-IR spectro- and 4 were collected and processed using the CrysAlisPro soft-
photometer. Elemental analyses were performed by Medac ware (Rigaku, 2012). Empirical absorption corrections were
Ltd, Surrey, UK. Mass spectrum was recorded on a Waters performed using spherical harmonics, implemented in
Mass Spectrometer with the MALDI Micro MX module. The SCALE3 ABSPACK scaling algorithm in the CrysAlisPro soft-
complex [Ce(LOEt)Cl3] was prepared according to the literature ware suite. Structure solution and refinement for all complexes
method.10 All other reagents were purchased from Aldrich Ltd were performed using the Olex2 software package17 (which
and used as received.
embedded SHELXTL18). All the structures were solved by direct
Reaction of 1 with [Ag2MoO4]. To a solution of 1 (80.5 mg, methods, expanded by difference Fourier syntheses and
0.103 mmol) in MeCN (5 mL) was added a suspension of refined by full matrix least-squares on F2. All non-hydrogen
[Ag2MoO4] (58 mg, 0.155 mmol) in MeCN (5 mL) at −30 °C, atoms were refined anisotropically with a riding model for the
and the reaction mixture was stirred at room temperature over- hydrogen atoms except noted separately. All the pictures of
night. The AgCl formed was filtered off and the volatiles were molecules were made using ORTEP. Polyhedron representation
removed in vacuo. The orange residue was extracted with was made using Diamond 3.19 CCDC 1006652 and 1451006
CH2Cl2, Recrystallization from CH2Cl2/Et2O at −18 °C afforded contain the supplementary crystallography data for complexes
orange single crystals that were identified as the previously 3 and 4 respectively.
This journal is © The Royal Society of Chemistry 2016
Dalton Trans.

View Article Online
Paper
Dalton Transactions
S. Shaikhutdinov, H. Schubert, H.-J. Freund, R. Schlogl,
J. Sauer and R. Schomacker, J. Catal., 2012, 296, 120.
Table 3 Crystallographic data and experimental details for 3 and 4
4·0.5H2O,
4 (a) T. Kropp, J. Paier and J. Sauer, J. Am. Chem. Soc., 2014,
136, 14616; (b) M. Baron, H. Abbott, O. Bondarchuk,
D. Stacchiola, A. Uhl, S. Shaikhutdinov, H.-J. Freund,
C. Popa, M. V. Ganduglia-Pirovano and J. Sauer, Angew.
Chem., Int. Ed., 2009, 48, 8006; (c) M. V. Ganduglia-
Pirovano, C. Popa, J. Sauer, H. L. Abbott, A. Uhl, M. Baron,
D. Stacchiola, O. Bondarchuk, S. Shaikhutdinov and
H.-J. Freund, J. Am. Chem. Soc., 2010, 132, 2345;
(d) J.-B. Ma, Z. Yuan, J.-H. Meng, Q.-Y. Liu and S.-G. He,
ChemPhysChem, 2014, 15, 4117; (e) J.-B. Ma, Z. Yuan,
J.-H. Meng and S.-G. He, Dalton Trans., 2015, 44, 3128;
(f) M. V. Ganduglia-Pirovano, Catal. Today, 2015, 253, 20.
5 (a) L. G. Hubert-Pfalzgraf, C. Sirio and C. Bois, Polyhedron,
1998, 17, 821; (b) L. G. Hubert-Pfalzgraf, V. Abada and
3·4MeCN
2/3C6H12, 1/3C6H14
Formula
Fw
Crystal system
Space group
a (Å)
b (Å)
c (Å)
C42H86Ce2Co2N4O44P6Re6 C74H153.67Ce4Co4O53.5P12V4
3052.27
Monoclinic
P21
12.9080(2)
24.6246(4)
13.3941(2)
90
97.8540(10)
90
4217.44(11)
2
2.404
173
2856
29.334
25 100
12 659
3271.23
Monoclinic
C2/c
32.0185(8)
17.4325(4)
24.4487(7)
90
107.710(3)
90
12 999.6(6)
4
1.671
99.9(4)
6551
18.831
35 670
11 528
α, (°)
β, (°)
γ, (°)
V (Å3)
Z
ρcalcd (g cm−1
T (K)
)
F(000)
μ (mm−1
)
No. of reflns
No. of indep
reflns
J.
Vaissermann,
Polyhedron,
1999,
18,
3497;
(c) A. J. Tasiopoulos, W. Wernsdorfer, B. Moulton,
M. J. Zaworotko and G. Christou, J. Am. Chem. Soc., 2003,
125, 15274; (d) A. Mishra, A. J. Tasiopoulos,
W. Wernsdorfer, E. E. Moushi, B. Moulton,
M. J. Zaworotko, K. A. Abboud and G. Christou, Inorg.
Chem., 2008, 47, 4832; (e) V. Mereacre, A. M. Ako,
M. N. Akhtar, A. Lindemann, C. E. Anson and A. K. Powell,
Helv. Chim. Acta, 2009, 92, 2507; (f) A. J. Tasiopoulos,
T. A. O’Brien, K. A. Abboud and G. Christou, Angew. Chem.,
Int. Ed., 2004, 43, 345; (g) A. J. Tasiopoulos, P. L. Milligan,
Jr., K. A. Abboud, T. A. O’Brien and G. Christou, Inorg.
Chem., 2007, 46, 9678; (h) M. Wang, D. Q. Yuan, C.-B. Ma,
M.-J. Yuan, M.-Q. Hu, N. Li, H. Chen, C.-N. Chen and
Q. T. Liu, Dalton Trans., 2010, 39, 7276; (i) S. Giessmann,
S. Blaurock, V. Lorenz and F. T. Edelmann, Inorg. Chem.,
2007, 46, 8100; ( j) M. P. Coles, P. B. Hitchcock,
A. V. Khvostov, M. F. Lappert, Z. Li and A. V. Protchenko,
Dalton Trans., 2010, 39, 6780; (k) G.-C. Wang, H. H. Y. Sung,
F.-R. Dai, W.-H. Chiu, W.-Y. Wong, I. D. Williams and
W.-H. Leung, Inorg. Chem., 2013, 52, 2556.
Rint
0.0533
1.004
0.0529, 0.1333
0.0464
1.003
0.0426, 0.1135
GoFa
R1 b, wR2
c
(I > 2σ(I))
R1, wR2
(all data)
0.0553, 0.1357
0.0584, 0.1222
a GoF = [∑w(|Fo| − |Fc|)2/(Nobs − Nparam)]1/2
.
b R1 = ∑||Fo| − |Fc||/∑|Fo|.
c wR2 = [∑w(|Fo2| − |Fc2|)2/∑w|Fo2|2]1/2
.
Acknowledgements
Financial support from the Hong Kong Research Grants
Council (project no. 603111, 602913 and 16301815) and the
Hong Kong University of Science and Technology is gratefully
acknowledged. We thank Mr Ka-Chun Au-Yeung for assistance
in the XPS study.
6 (a) D. D. Dexter and J. V. Silverto, J. Am. Chem. Soc., 1968,
90, 3589; (b) J. Iball, J. N. Low and T. J. R. Weakley, J. Chem.
Soc., Dalton Trans., 1974, 2021; (c) R. Copping, L. Jonasson,
A. J. Gaunt, D. Drennan, D. Collison, M. Helliwell,
R. J. Pirttijarvi, C. J. Jones, A. Huguet, D. C. Apperley,
N. Kaltsoyannis and I. May, Inorg. Chem., 2008, 47, 5787.
7 (a) D. Liu, Y. Lu, H.-Q. Tan, T.-T. Wang and E.-B. Wang,
Cryst. Growth Des., 2015, 15, 103; (b) M. Nishio, S. Inami
and Y. Hayashi, Eur. J. Inorg. Chem., 2013, 1876;
(c) A. Seliverstov and C. Streb, Chem. – Eur. J., 2014, 20,
9733; (d) A. Seliverstov and C. Streb, Chem. Commun., 2014,
50, 1827.
Notes and references
1 (a) A. Trovarelli, Catal. Rev. Sci. Eng., 1996, 38, 439;
(b) A. Trovarelli, C. de Leitenburg, M. Boaro and
G. Dolcetti, Catal. Today, 1999, 50, 353; (c) R. D. Monte and
J. Kašpar, Top. Catal., 2004, 28, 47; (d) Q. Fu, H. Saltsburg
and M. Flytzani-Stephanopoulos, Science, 2003, 301, 935;
(e) D. Pierre, W. Deng and M. Flytzani-Stepha-nopoulos,
Top. Catal., 2007, 46, 363; (f) C. Serre, F. Garin, G. Belot
and G. Maire, J. Catal., 1993, 141, 9; (g) S. Imamura, in
Catalysis by Ceria and Related Materials, ed. A. Trovarelli,
Imperial College Press, London, 2002, p. 431.Catalysis by
Ceria and Related Materials, ed. A. Trovarelli and P. Forna-
siero, Imperial College Press, London, 2nd edn, 2013.
2 J. Paier, C. Penschke and J. Sauer, Chem. Rev., 2013, 113,
3949.
8 W.-H. Leung, Q.-F. Zhang and X.-Y. Yi, Coord. Chem. Rev.,
2007, 251, 2266.
9 (a) Q.-F. Zhang, T. C. H. Lam, E. Y. Y. Chan, S. M.-F. Lo,
I. D. Williams and W.-H. Leung, Angew. Chem., Int. Ed.,
2004, 43, 1715; (b) Q.-F. Zhang, T. C. H. Lam, X.-Y. Yi,
E. Y. Y. Chan, W.-Y. Yeung, H. H.-Y. Sung, I. D. Williams
and W.-H. Leung, Chem. – Eur. J., 2004, 11, 101; (c) X.-Y. Yi,
3 (a) A. Khodakov, B. Olthof, A. T. Bell and E. Iglesia,
J. Catal., 1999, 181, 205; (b) B. Beck, M. Harth,
N. G. Hamilton, C. Carrero, J. J. Uhlrich, A. Trunschke,
Dalton Trans.
This journal is © The Royal Society of Chemistry 2016

View Article Online
Dalton Transactions
Paper
H. H.-Y. Sung, I. D. Williams and W.-H. Leung, Chem.
Commun., 2008, 3269; (d) X.-Y. Yi, T. C. H. Lam,
I. D. Williams and W.-H. Leung, Inorg. Chem., 2010, 49,
2232.
(b) M. Casny and D. Rehder, Dalton Trans., 2004, 5, 839;
(c) N. Hall, M. Orio, A. Jorge-Robin, B. Gennaro, C. Marchi-
Delapierre and C. Duboc, Inorg. Chem., 2013, 52, 13424;
(d) P. Adäo, J. C. Pessoa, R. T. Henriques, M. L. Kuznetsov,
F. Avecilla, M. R. Maurya, U. Kumar and I. Correia, Inorg.
Chem., 2009, 48, 3542; (e) M. R. Maurya, P. Saini, A. Kumar
and J. C. Pessoa, Eur. J. Inorg. Chem., 2011, 4846;
(f) A. Colettia, P. Gallonia, A. Sartorelb, V. Contea and
B. Floris, Catal. Today, 2012, 192, 44.
10 G.-C. Wang, Y.-M. So, K.-L. Wong, K.-C. Au-Yeung, H.
H.-Y. Sung, I. D. Williams and W.-H. Leung, Chem. – Eur. J.,
2015, 21, 16126.
11 Y.-M. So, G.-C. Wang, Y. Li, H. H.-Y. Sung, I. D. Williams,
Z. Lin and W.-H. Leung, Angew. Chem., Int. Ed., 2014, 53,
1626.
16 (a) A. Butler, M. J. Clague and G. E. Meister, Chem. Rev.,
1994, 94, 625; (b) A. G. J. Ligtenbarg, R. Hage and
B. L. Feringa, Coord. Chem. Rev., 2003, 237, 89;
(c) M. Vandichel, K. Leus, P. V. D. Voort, M. Waroquier and
V. V. Speybroeck, J. Catal., 2012, 294, 1.
12 31P NMR signals for diamagnetic CeIV–LOEt complexes are
usually observed at δ 115–125 ppm in CDCl3, whereas those
for the CeIII analogues such as [CeIII(LOEt)2(H2O)2Cl] occur
at δ 150–170 ppm. In addition, 1H NMR spectra of CeIII–
LOEt complexes exhibit paramagnetically shifted broad 17 O. V. Dolomanov, L. J. Bourhis, R. J. Gildea, J. A. K. Howard
signals.
and H. Puschmann, J. Appl. Crystallogr., 2009, 42, 339.
OLEX2: A complete structure solution, refinement and ana-
lysis program.
13 D. Rehder, Coord. Chem. Rev., 2008, 252, 2209.
14 (a) A. S. Wills, VaList version 4.07, U.C.L., London, 2010;
(b) I. D. Brown and D. Altermatt, Acta Crystallogr., Sect. B: 18 G. M. Sheldrick, Acta Crystallogr., Sect. A: Fundam. Crystal-
Struct. Sci., 1985, 41, 244; (c) A. Trzesowska, R. Kruszynski logr., 2008, 64, 112.
and T. Bartczak, Acta Crystallogr., Sect. B: Struct. Sci., 2006, 19 Diamond - Crystal and Molecular Structure Visualization
62, 745.
Crystal Impact - Dr. H. Putz & Dr. K. Brandenburg GbR,
Kreuzherrenstr. 102, 53227 Bonn, Germany, http://www.
crystalimpact.com/diamond.
15 (a) M. Mba, M. Pontini, S. Lovat, C. Zonta, G. Bernardinelli,
P. E. Kundig and G. Licini, Inorg. Chem., 2008, 47, 8616;
This journal is © The Royal Society of Chemistry 2016
Dalton Trans.