A tetranuclear diphenyltin(IV) complex and its catalytic activity in the aerobic Baeyer-Villiger oxidation of cyclohexanone

Article abstract of DOI:10.1016/j.jorganchem.2017.12.040

The synthesis and crystal structure of the new tetranuclear diphenyltin(IV) compound [{SnPh₂}{SnPh₂(OCOMe)}(μ₂-OMe)(μ₃-O)]₂ (1), generated from the reaction of [SnPh₂Cl₂] and NaOCOMe, are described. Single crystal X-ray diffraction revealed that 1 is a tetranuclear complex with four diphenyltin(IV) units which are inter-connected by μ₃-oxido and μ₂-methoxido bridges leading to a ladder-like Sn₄O₄ cluster. Structural comparison with related compounds found in crystallographic data base (CSD) attests that such tetranuclear systems are more abundant than related oxido bridged dimers or trimers. Complex 1 efficiently catalyzes the aerobic Baeyer-Villiger oxidation of cyclohexanone to ε-caprolactone, under mild conditions. With the sacrificial benzaldehyde method, quantitative conversion was observed in just 30 min with a remarkable selectivity.

Full text of DOI:10.1016/j.jorganchem.2017.12.040

Accepted Manuscript
A tetranuclear diphenyltin(IV) complex and its catalytic activity in the aerobic Baeyer-
Villiger oxidation of cyclohexanone
Susanta Hazra, Nuno M.R. Martins, Kamran Mahmudov, Fedor I. Zubkov, M.F.C.
Guedes da Silva, Armando J.L. Pombeiro
PII:
S0022-328X(17)30738-6
10.1016/j.jorganchem.2017.12.040
JOM 20235
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 9 October 2017
Revised Date: 27 December 2017
Accepted Date: 30 December 2017
Please cite this article as: S. Hazra, N.M.R. Martins, K. Mahmudov, F.I. Zubkov, M.F.C. Guedes da
Silva, A.J.L. Pombeiro, A tetranuclear diphenyltin(IV) complex and its catalytic activity in the aerobic
Baeyer-Villiger oxidation of cyclohexanone, Journal of Organometallic Chemistry (2018), doi: 10.1016/
j.jorganchem.2017.12.040.
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ACCEPTED MANUSCRIPT
A tetranuclear diphenyltin(IV) complex and its catalytic activity in the
aerobic Baeyer-Villiger oxidation of cyclohexanone†
Susanta Hazra,*a Nuno M. R. Martins,a Kamran Mahmudov,a,b Fedor I. Zubkov,c M. Fátima C. Guedes da
Silvaa and Armando J. L. Pombeiro*a
aCentro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049–001
Lisboa, Portugal. E-mail: h.susanta@gmail.com and pombeiro@tecnico.ulisboa.pt
bDepartment of Ecology and Soil Sciences, Baku State University, Z. Xalilov Str. 23, Az 1148 Baku, Azerbaijan
cOrganic Chemistry Department, RUDN University, 6 Miklukho-Maklaya Str., Moscow 117198, Russian Federation
†The authors from Azerbaijan, Portugal and Russia dedicate this work to Prof. Irina Beletskaya on the occasion of
her 85th birthday, who is well known with her excellent work in the chemistry of organometallic compounds and
catalysis.
Abstract
The synthesis and crystal structure of the new tetranuclear diphenyltin(IV) compound
[{SnPh2}{SnPh2(OCOMe)}(µ2-OMe)(µ3-O)]2 (1), generated from the reaction of [SnPh2Cl2] and
NaOCOMe, are described. Single crystal X-ray diffraction revealed that 1 is a tetranuclear complex with
four diphenyltin(IV) units which are inter-connected by µ3-oxido and µ2-methoxido bridges leading to a
ladder-like Sn4O4 cluster. Structural comparison with related compounds found in crystallographic data
base (CSD) attests that such tetranuclear systems are more abundant than related oxido bridged dimers or
trimers. Complex 1 efficiently catalyzes the aerobic Baeyer-Villiger oxidation of cyclohexanone to ε-
caprolactone, under mild conditions. With the sacrificial benzaldehyde method, quantitative conversion
was observed in just 30 min with a remarkable selectivity.
Keywords: Tetranuclear diphenyltin(IV), Single crystal X-ray, Ladder-like, Baeyer-Villiger oxidation,
Sacrificial aldehyde, Cyclohexanone.
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1. Introduction
Organotin chemistry initiated a long time ago, in the middle of the XIX century, when
diethyltin diiodide, [(C2H5)2SnI2], was characterized by Frankland [1,2]. The field has
experienced a great development, enriched by a wide variety of structures and biological
applications of organotin complexes [3-6]. Examples include their usage as heat stabilizers in
industry [7], as catalysts [8-13] for several important organic transformations, such as
esterification [8], production of urethane foams and elastomers [9], polymerization [10],
epoxidation of olefins [11], conversion of 1,2-diols into α-hydroxyketones [12], and curing of
silicones [13].
The Baeyer-Villiger (BV) oxidation provides an important route to synthesize esters and
lactones which are intermediates for some valuable products [14-20]. Usually,
peroxyacids or peroxides are used as oxidants for this conversion [14,15], and approaches to
perform the BV oxidation under eco-friendly conditions have been reported [14d-g,16,17]. A
catalyst is required when mild oxidants, such as molecular oxygen with benzaldehyde (sacrificial
method) [17] or aqueous hydrogen peroxide [15b-i,18-20], are used, and tin-containing catalytic
systems [17i,18-20] have been investigated. For example, homo- or heterogenous tin containing
catalysts [14e,16e,17i,18-20], such as some tin-based fluorous biphase systems [18a,b], zeolites
[14e,18c-f] and mesoporous composites [19a-e], clays [19f-k], metal oxide composites [16e,
20a-d] or polymer supported [20f-j], have been studied in this oxidation reaction. Very recently,
we have reported that the sacrificial aldehyde method is better than the use of aqueous H2O2 for
heterometallic 3d-tin(IV) systems [17i]. We now extend the study to test a new tin compound as
a catalyst for the aerobic BV oxidation, particularly in the presence of benzaldehyde.
Accordingly, we report the synthesis, crystal structure and catalytic behaviour in the BV aerobic
oxidation of cyclohexanone (using sacrificial benzaldehyde) of the newly synthesized ladder-like
tetranuclear diphenyltin(IV) compound [{SnPh2}{SnPh2(OCOMe)}(µ2-OMe)(µ3-O)]2 (1).
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Scheme 1. Synthesis of [{SnPh2}{SnPh2(OCOMe)}(µ2-OMe)(µ3-O)]2 (1).
2. Experimental section
2.1. Materials and physical methods
All the reagents and solvents were purchased from commercial sources and used as
1
received. H (300 MHz), 13C (75.45 MHz) and 119Sn NMR spectra were run on a Bruker 300
MHz spectrometer using tetramethylsilane [Si(CH3)4] as internal reference for proton and carbon
and, [Sn(CH3)4] as external standard for tin. The infrared spectrum (4000–400 cm–1) was
recorded on a Bruker Vertex 70 instrument in KBr pellets (abbreviations: s = strong, m =
medium and w = weak) while elemental (C, H and N) analysis was performed by the
Microanalytical Service of the Instituto Superior Técnico.
Gas chromatographic (GC) measurements were carried out using a FISONS Instruments
GC 8000 series gas chromatograph with a FID detector and a capillary column (DB-WAX,
column length: 30 m; internal diameter: 0.32 mm). The temperature of injection was 240 °C. The
initial temperature was maintained at 100 ◦C for 1 min, then raised 10 ºC/min up to 180 ºC, and
held at this temperature for 1 min. Helium was used as the carrier gas.
2.2. Synthesis of [{SnPh2}{SnPh2(OCOMe)}(µ2-OMe)(µ3-O)]2 (1)
To a hot and stirred methanol (5 mL) solution of NaOCOMe (0.082 g, 1.0 mmol) was
added dropwise a methanol solution (5 mL) of [SnPh2Cl2] (0.344 g, 1.0 mmol). After stirring for
2 h, the resulted white suspension was dissolved by dropwise addition of acetone (10 mL). After
2 d, white X-ray quality crystals formed which were collected by filtration and washed with cold
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methanol. Yield: 0.173 g (53%). C54H52O8Sn4 (1303.71): calcd C 49.74, H 4.02%; found C
50.01, H 4.06%. IR data (KBr, cm–1): νas(COO) 1653s, νs(COO) 1403s, ν(O-Sn-O) 654m, ν(Sn-
C) 511m, ν(Sn-O) 437w. 1H NMR (300 MHz, CDCl3, δ ppm, Figure S2, SI): 7.32–8.11 (m, Ar-
13
H); 3.94 (s, 3H, CH3O), 2.24 (3H, s, CH3COO). C NMR (75.45 MHz, CDCl3, δ ppm, Figure
S3, SI): 172.7 (COO), 123.7–156.9 (Ar-C), 50.2 (CH3O), 21.3 (CH3COO). 119Sn NMR (300
MHz, CDCl3, δ ppm, Figure S4, SI): –45.09.
2.3. Crystal structure determination
The X-ray diffraction data were collected using a Bruker APEX-II PHOTON 100 with
graphite monochromated Mo-Kα radiation. Data were collected using omega scans of 0.5º per
frame, and a full sphere of data was obtained. Cell parameters were retrieved using Bruker
SMART [21a] software and refined using Bruker SAINT [21a] on all the observed reflections.
Absorption corrections were applied using SADABS [21a]. Structures were solved by direct
methods by using the SHELXS-2014 package [21b,c] and refined with SHELXL-2014.01
[21b,c]. Calculations were performed using the WinGX System-Version 2014.01 [21d]. All the
hydrogen atoms were inserted at geometrically calculated positions and refined using the riding-
model approximation. Least square refinements with anisotropic thermal motion parameters for
all the non-hydrogen atoms and isotropic for most of the remaining atoms were employed. The
final refinement converged to R1 (I >2σ(I)) value of 0.0258. Crystallographic data are presented
in Table S1 (Supplementary information).
2.4. Typical procedures and product analysis of BV oxidation
In a typical experiment, a mixture of catalyst precursor (8.3 mg, 6 × 10–3 mmol),
cyclohexanone (104 µL, 1 mmol), benzaldehyde (204 µL, 2.0 mmol) and 1,2-dichloroethane (1.0
mL) was taken in a 5 mL reaction flask equipped with a condenser and a balloon with air. The
reaction mixture was stirred at room temperature for 3 h. The reaction mixture was diluted by
adding acetonitrile (3.0 mL). Then, 50 µL of cycloheptanone (internal standard) was introduced
into the reaction mixture which was stirred for 10 min. A sample (0.5 µL) was taken from the
reaction mixture and analyzed by GC using the internal standard method. The product ɛ-
caprolactone was isolated by silica-gel column chromatography using ethyl acetate:n-hexane
(1:8).
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3. Results and discussion
3.1. Synthesis and characterization
The reaction of [SnPh2Cl2] with NaOCOMe in methanol-acetone (1:1) solvent mixture
produces the tetranuclear diphenyltin(IV) compound [{SnPh2}{SnPh2(OCOMe)}(µ2-OMe)(µ3-
O)]2 (1) (Scheme 2) in an average yield of 53%. However, firstly an identical procedure [17i]
used for the synthesis of heterometallic 3d-SnIV systems, attempting to obtain a Zn-Sn compound
from the reaction of the Schiff base N,N′-ethylenebis(salicylaldimine), Zn(OCOMe)2·2H2O and
[SnPh2Cl2], resulted in the title compound 1 in lower yield (15%) with some unidentified
impurities. Later, 1 was prepared from the reactants ([SnPh2Cl2] and NaOCOMe) which are
related to components of the final product. It was characterized by IR, NMR, elemental and
single crystal X-ray diffraction analyses. To be mentioned that the 1:2 ratio of NaOCOMe and
[SnPh2Cl2], as it is in the compound, produced a considerable lower yield (~ 29%) but higher
amount of NaOCOMe increased the yield (53%) of 1. The solvents (methanol and acetone) used
in the synthesis were not further purified and used as received. Two days of evaporation would
increase the ratio of water in the solvent mixture. Thus, the little amount of the generated NaCl
(~ 0.058 g) remains soluble and was not required to remove it from the reaction mixture.
In the IR spectrum, it exhibits two strong absorptions at 1653 and 1403 cm–1 assigned to
υ(OCO) vibrations. The presence of O–Sn–O, Sn–O and Sn–C is evidenced by the
medium/weak intense bands at 654, 511 and 437 cm–1, respectively, which are comparable to
other similar reported compounds [22,23].
3.2. Description of the crystal structure
The asymmetric unit of 1 comprises two types of tin moieties, {SnPh2} and
{SnPh2(OCOMe)}, bridged by an oxido and a methoxido; the structure expands to a tetranulear
unit in view of the proximity of an inversion centre, thus generating four Sn(IV) positions
interconnected by µ3-oxido and µ2-methoxido groups. The structure of 1 is depicted in Figure 1
and selected bond length and angles are listed in Table 1.
Both metal atoms of 1 present highly distorted C2O3 coordination environments with τ5
values of 0.38 (Sn1) and 0.27 (Sn2) [24], with two of the tin atoms linked to an Oacetate, an
Ometoxido and an Ooxido, and the other two bound to two Ooxido and to an Ometoxido. The Sn–Cphenyl
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bond distances are comparable, as well as the Cphenyl−Sn−Cphenyl bond angles (Table 1). However,
while the phenyl groups attached to Sn1 are nearly coplanar as shown by the angle of 8.84º
between their least square planes, those coupled to Sn2 are twisted by 25.52º. The Sn−O lengths
range from 2.024(2) to 2.229(2) Å, the longest one pertaining to a Sn−Omethoxido and the shortest
to a Sn−Ooxido. The Sn−O and Sn−C distances found in 1 are similar to those found in other
reported ladder-like tetranuclear diorganotin(IV) systems [22,23].
Figure 1. Ball and stick representation of 1 with partial atom labelling scheme. The symmetry
generated half of the compound is depicted in grey wireframe with the metal atoms as grey balls.
Hydrogen atoms are omitted for clarity. Symmetry code to generate equivalent atoms: i) 2–x, 2–
y, 1–z.
Figure 2. The ladder-like Sn4O4 metallacycle in 1. Cyan and red color balls present tin and
oxygen atoms, respectively.
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Table 1. Selected bond distances (Å) and angles (º) in 1. Symmetry as in the Figure 1.
between the l.s. planes of the
8.84 (Sn1), 25.53 (Sn2)
phenyl rings
between the l.s. planes of the
Sn4O4 metallacycle and phenyl
rings
72.62,85.16,86.01,89.08
Sn1···Sn2 (closet)
Sn2···Sn2’
3.390
3.345
6.526
Sn1···Sn2 (terminal)
2.116(3), 2.119(3), 2.132(3),
2.122(3)
Sn−Cphenyl
2.024(2), 2.048(2), 2.140(2)
2.229(2), 2.125(2)
2.129(2)
Sn−Ooxido
Sn−Omethoxido
Sn−Oacetato
106.01(9), 112.66(10),
140.91(11)
Sn−Ooxido−Sn
102.21(9)
132.62(14),130.75(13)
155.22(9)
Sn−Omethoxido−Sn
Cphenyl−Sn−Cphenyl
Omethoxido−Sn−Oacetato
The crystal structure of 1 is stabilized by weak C–H···O interactions, one intramolecular
(D···A = 3.157 Å and
D–H···A = 137 º) and involving the coordinated Oacetate atom, and one
intermolecular (D···A = 3.394 Å and D–H···A = 179 º) that connects the non-coordinated
Oacetate atom to a Hphenyl of a vicinal molecule. This latter contact expands the structure into a 1D
infinite chain.
Figure 3. C–H···O supported one dimensional structure in 1.
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3.3. Abundancy and stability of compounds with ladder-like (R2Sn)4O2(O–X)2 framework
Our search in the CSD (Crystallographic Structural Database, ConQuest Version 1.19)
[22] revealed that the tin(IV) cation has a high tendency to form and stabilize oxido-bridged
ladder-like tetranuclear moieties of the type (R2Sn)4O2(O–X)2 [23] (Figure 4) as observed in 1,
followed by systems with a trinuclear (R2Sn)3O structure and then by those with oxo-bridged
dinuclear (R2Sn)2O2 ones (Figure 4). The observed trend is an indication of the preferential
stability of tetranuclear oxido-tin(IV) systems over di- (14) or trinuclear (26) arrangements,
which is uncommon in structures involving other metals. However, the tetranuclear ladder-like
frame can be viewed as a combination of both oxido-bridged dimeric (red marked shape, Figure
4) and trimeric (green marked shape, Figure 4) frameworks.
Among these tetranuclear assemblies, dibutyl- and dimethyltin(IV) constitute the
majority (60 and 40 structures, respectively), while compounds with pentyl-, hexyl- or heptyl
groups were not found.
Figure 4. Numbers of compounds with oxido-bridged ladder-like tetranuclear (R2Sn)4O2(O–X),
oxo-bridged dinuclear (R2Sn)2O2 and trinuclear (R2Sn)3O frameworks that could be found in
CSD. R = Me, Et, Pr, i-Pr, n-Bu, Ph, etc. X = no atom, H, Me, Et, i-Pr, etc.
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3.4. Baeyer-Villiger oxidation of cyclohexanone
Cyclohexanone was selected as model substrate to study the catalytic behaviour of 1
towards the Baeyer-Villiger (BV) reaction. The corresponding product, -caprolactone, (Scheme
ε
2) is a common intermediate with significance to the production of high valuable fine chemicals
[14b-d]. In the present study, O2/benzaldehyde was used as oxidant-sacrificial aldehyde pair
since it has been found to be effective in tin(IV)-based catalytic systems [17h,i]. 1,2-
dichloroethane was chosen as solvent due to its resistance to oxidizing agents and common use
in BV reactions [17h,i,25].
Scheme 2. Aerobic BV oxidation of cyclohexanone into
ε-caprolactone catalyzed by 1.
Table 2. Baeyer–Villiger oxidation of cyclohexanone.a
Conversionb /%
Yieldc /%
Selectivityd /%
TONe
TOFf
Precatalyst
Entry
Conc./mol%
1
1
2
3
4
5
6
0.1
0.3
0.6
1.0
0.1
0.1
84.3
91.1
92.6
93.3
23.2
13.4
81.8
85.5
88.7
89.2
14.9
11.1
97.0
93.9
95.8
95.6
64.2
82.8
818
285
148
89.2
149
111
273
95.0
49.3
29.7
49.7
37.0
1
1
1
[SnPh2Cl2]
NaOCOMe
[SnPh2Cl2] +
NaOCOMe
-
7
0.05+0.05
17.4
14.6
12.3
11.2
70.7
76.7
123
-
41.0
-
8
-
aReaction conditions (unless stated otherwise): cyclohexanone, 1 mmol; benzaldehyde, 2 mmol; dioxygen, 1 atm; 1,2-
dichloroethane, 1.0 mL; reaction temperature, 25 ˚C; reaction time, 3 h. Yield and TON determined by GC analysis. bConversion
of cyclohexanone. cMolar yield (%) based on the cyclohexanone, i.e. moles of lactone per 100 moles of cyclohexanone.
dSelectivity based on ε-caprolactone (mols of lactone per 100 mol of converted cyclohexanone). Turnover number (moles of
product per mole of precatalyst). fTurnover frequency = TON per hour.
e
The catalyst loading was screened under fixed reaction conditions at room temperature
(25 ˚C) (Table 2, entries 1-4, Figure 5). High selectivity (94-97 %) was achieved in all runs.
Maximum yield (89%, entry 4, Table 2) of
ε
-caprolactone was obtained with 1.0 mol% (based on
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cyclohexanone) of 1. Beyond this catalyst loading, no significant increment was observed. The
maximum yield was almost reached for 0.6 mol% of 1, and thus this catalyst loading was used in
the following reaction studies.
Figure 5. Aerobic Baeyer–Villiger oxidation of cyclohexanone at various catalyst
concentrations. Reaction conditions: as stated in the footnote of Table 2.
The influence of temperature is indicated in Table 3 and Figure 6. Performing the
catalytic reaction at a slightly higher temperature (40 ˚C) than room temperature, with 0.6 mol%
catalyst loading, leads to the almost quantitative formation of ε-caprolactone (99% yield, entry 6,
Table 3) in just 30 min, beyond which the yield remains almost invariable up to 3 h. Moreover, a
considerable yield (74.3%, entry 5, Table 3) was achieved in just 15 min at 40 ˚C. Further
increasing the temperature (60 ˚C) produces similar results as those obtained at 40 ˚C for the first
hour, e.g., reaching ε-caprolactone yields of 82.5% (entry 9, Table 3) and 98.1% (entry 10, Table
3) in 15 or 30 min, respectively. However, running the reaction for a longer period than 1 h leads
to loss of the yield (69.2%, after 3 h, entry 12, Table 3), which can be attributed to the
decomposition of ε-caprolactone as observed in other cases [17h,i].
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Table 3. Baeyer–Villiger oxidation of cyclohexanone by 1 at different temperatures.a
Time /min
Entry Temp. /˚C
1
Conversionb /%
Yieldc /%
Selectivityd /%
TONe
TOFf
15
30
53.7
61.1
77.6
94.6
76.3
99.8
99.9
99.9
84.9
99.9
99.9
99.9
48.6
58.1
72.3
88.7
74.3
99.0
98.6
98.4
82.5
98.1
97.6
69.2
90.5
95.1
93.2
93.8
97.4
99.9
99.6
99.4
97.2
99.1
98.6
69.8
81.0
96.8
121
148
124
165
164
164
138
164
163
115
324
194
121
49.3
495
330
164
54.7
550
327
163
38.4
2
25
3
60
4
5
180
15
6
30
40
7
60
8
9
180
15
10
60
11
30
60
12
180
aReaction conditions (unless stated otherwise): catalyst 1, 0.6 mol% vs. substrate; cyclohexanone, 1 mmol; benzaldehyde, 2
mmol; dioxygen, 1 atm; 1,2-dichloroethane, 1.0 mL. Yield and TON determined by GC analysis. bConversion of cyclohexanone.
d
cMolar yield (%) based on the cyclohexanone, i.e. moles of lactone per 100 moles of cyclohexanone. Selectivity based on ε-
e
caprolactone (mols of lactone per 100 mol of converted cyclohexanone). Turnover number (moles of product per mole of
precatalyst). fTurnover frequency = TON per hour.
In the absence of any catalyst (blank experiment) a very small amount of ε-caprolactone
(11%) was detected after 3 h (Table 2, entry 8). Very similar results (only up to ~15%, entries 5-
7, Table 2) were also observed when using [SnPh2Cl2] and sodium acetate (NaOCOMe) or an
equimolar mixture of both.
The tetratin(IV) catalyst 1 is proved to be highly efficient and very selective towards the
BV reaction of cyclohexanone at low temperature. However, considering the tetranuclear feature
of the metal complex under optimal reaction conditions to afford the highest yield (0.6 mol % vs.
1
cyclohexanone, 40 ˚C, 0.5 h), the original TON and TOF of ca. 165 and 330 h− , respectively
1
(entry 6, Table 3) are virtually reduced by four (41 and 83 h− per Sn atom, respectively).
Nevertheless, the TON and TOF values per Sn are considerably higher than those of [SnPh2Cl2],
as shown by comparing entries 1 and 5 of Table 2, at 25 ˚C (e.g., TON values of 818/4 = 204 for
1 and 149 for [SnPh2Cl2]).
Several tin catalytic systems [14-20] have been used in the BV oxidation of
cyclohexanone to ε-caprolactone and some of them are listed in Table 4. These selected
examples show that hydrogen peroxide is an effective oxidant which is used more frequently
than the dioxygen-benzaldehyde pair. The most effective tin-based catalysts are usually
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heterogeneous, allowing the recovery of the catalyst after the reaction and its possible reuse.
Although our catalytic system (1), being homogenous, has the limitation of recovery, it displays
higher activity (reaching full conversion just after 30 mins) at low temperature (40 ˚C), what
constitutes advantages over other catalytic systems.
Table 4. Comparison of 1 with other tin catalytic systems in BV oxidation of cyclohexanone.
Catalyst
Conversion of
Selectivity of ε-
Oxidant
Reference
cyclohexanone / % caprolactone / %
Sn-zeolite beta
52
75
83
30
19
91
53
90-99
87.6
96
78.5-83.1
91
83
30-73
13
50
21
20
36
>98
90
18
>95
91
>99
>99
-
35% H2O2
14e
14h
15e
15f
15g
15h
15i
Sn(salen)-NaYa
70% t-BuOOH
35% H2O2
30% H2O2
67% H2O2
30% H2O2
35% H2O2
O2/benzaldehyde 16e
O2/benzaldehyde 16e
O2/benzaldehyde 17f
O2/benzaldehyde 17i
O2/benzaldehyde 17l
O2/benzaldehyde 17m
35% H2O2
35% H2O2
35% H2O2
35% H2O2
35% H2O2
35% H2O2
30% H2O2
30% H2O2
30% H2O2
HBEA 973b
Sn-doped hydrotalcitec
Sn-Beta-Trans-60
SnO2 (15 wt%)/GOd
Sn-MesoSB-60e
Fe-Sn-O mixed oxides
Mg-Sn-O mixed oxides
-
f
FeTPPCl/SnO2
>99
85-89
93
>99
62-97
57
60
59
64
97
Co-Sn, Cu-Sn and Ni-Sng
15Sn–TiO2
SnTPP/4A-MSh
Sn[N(SO2C8F17)2]4i
Bulk Sn-MFIj
18a,b
18d
18d
18d
18d
19a
19d
19f
Bulk Sn-Betak
Sn-MCM-41l
Sn-MFI-ns-16j
Sn-MCM-41(9% SnO2)l
Sn-SBA-15(50)m
Sn-palygorskiten
Montmorilloniteo
supported SnCl2
Sn(0.77)/TNp
70
16
100
86
90
100
19h
100
99.8
78
99
30% H2O2
O2/benzaldehyde This work
19k
1
b
asalen = N,N'-bis(salicylaldehyde)-1,2-ethylenediamine); HBEA = ammonium exchanged and thermally treated
c
beta zeolite; hydrotalcite = a layered double hydroxide of general formula Mg6Al2CO3(OH)16·4H2O; dGO =
e
f
g
graphene oxide; MesoSB = mesoporous silica beads; TPP = tetraphenylporphyrin; heterometallic compounds of
h
i
j
salen; 4A-MS = 4Å molecular sieve; HN(SO2C8F17)2 = bis(perfluorooctanesulfonyl)amide; MFI = mordenite
framework inverted (zeolite); kSn-Beta = stannosilicate material, with the BEA-type zeolite topology and
tetrahedrally coordinated Sn4+ in the framework; MCM-41= mobil composition of matter no. 41, a mesoporous
l
m
material with a hierarchical structure from a family of silicate and alumosilicate solids; SBA-15 = Santa Barbara
n
amorphous-15, a mesoporous form of silica nanoparticles; palygorskite = magnesium aluminium phyllosilicate;
omontmorillonite = a very soft phyllosilicate group of minerals that form when they precipitate from water solution
as microscopic crystals, known as clay; pTN = tainiolite, [(Mg2Li)(Si4O10)F2].
12

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Figure 6. Aerobic Baeyer–Villiger oxidation of cyclohexanone at 25, 40 and 60 ºC and along
time. Reaction conditions: as stated in the caption of Table 3.
A radical mechanism is expected for the current catalytic reaction as the oxidation of
cyclohexanone is completely inhibited in the presence of a radical trap (diphenylamine). As
presented in Scheme 3, the acylperoxyl radical generated from the metal-promoted reaction of
benzaldehyde and O2, could abstract hydrogen from the aldehyde to give peroxybenzoic acid.
The nucleophilic attack of this acid to metal-activated cyclohexanone produces the Criegee-type
adduct which, upon heterolytic O–O bond cleavage, would convert to the ε-caprolactone and
benzoic acid [17h-k].
O
O
O
O
O2
[Sn]
PhCHO
- PhCO
Ph
OOH
Ph
Ph
H
OO
Ph
[Sn]
O
O
O
H
O
Ph
O
O
- PhCOOH
O
Scheme 3. Possible mechanism for the aerobic Baeyer-Villiger oxidation of cyclohexanone with
benzaldehyde catalyzed by [{SnPh2}{SnPh2(OCOMe)}(µ2-OMe)(µ3-O)]2 (1).
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4. Conclusion
We have isolated a new ladder-like diphenyltin(IV) based tetranuclear complex,
[{SnPh2}{SnPh2(OCOCH3)}(µ2-OMe)(µ3-O)]2 (1), which contains different types of O-ligands
(oxido, methoxido and acetato) in addition to phenyl. Crystal structure analysis reveals that four
diphenyltin(IV) moieties, interconnected by µ3-oxido (O2–) and µ2-methoxido (CH3O–) bridges,
form a ladder-like Sn4O4 metallacycle which is, basically, a combined framework of related oxo-
bridged dimers (µ2-O) and trimers (µ3-O). A comparison with related compounds found in
crystallographic database (CSD) revealed that such tetranuclear systems are more abundant than
the oxido bridged dimers or trimers, which is unusual in the case of other metal systems.
Following the sacrificial aldehyde method (O2/benzaldehyde as oxidant-sacrificial
aldehyde pair), compound 1 efficiently catalyzes the Baeyer-Villiger oxidation of cyclohexanone
to ɛ-caprolactone in 30 min only (at 40 ˚C). Besides the remarkably high selectivity (99.9%)
achieved by 1, other attractive features are: the use of molecular oxygen, low catalyst loading,
mild conditions and short reaction time. According to our knowledge, this is the first time a
ladder-like tetranuclear system, such as 1, has been studied for the BV oxidation. Extension of
the study to related Sn compounds is underway.
Acknowledgements
Financial supports from the Fundação para a Ciência e a Tecnologia (FCT), Portugal, for
fellowships (SFRH/BPD/78264/2011 and SFRH/BD/52371/2013 to S.H. and N.M.R.M.,
respectively) and for the UID/QUI/00100/2013 and PTDC/QEQ-QIN/3967/2014 projects are
gratefully acknowledged. This work also was supported by the ''RUDN University Program 5-
100''.
Appendix A. Supplementary data
Table S1, crystallographic data for compound 1. CCDC 1560947 for 1 contain the supplementary crystallographic
data for this paper. The CIF data can also be obtained free of charge from the Cambridge Crystallographic Data
Centre via http://www.ccdc.cam.ac.uk/data_request/cif or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223 336 033 or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found in the online version.
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Highlights
ꢀ A tetranuclear diphenyltin(IV) complex was synthesized.
ꢀ The complex was characterized by X-ray, NMR and FT-IR spectra.
ꢀ A ladder-like Sn4O4 cluster supported by oxido and methoxido bridges.
ꢀ It catalyzes the aerobic Baeyer-Villiger oxidation of cyclohexanone to ε-caprolactone.
ꢀ With sacrificial benzaldehyde method quantitative conversion was observed in 30 min.