Novel oxo-peroxo molybdenum(VI) complexes incorporating 8-quinolinol: Synthesis, structure and catalytic uses in the environmentally benign and cost-effective oxidation method of methyl benzenes: Ar(CH3), (n = 1, 2)

Article abstract of DOI:10.1039/a904440j

A hitherto unknown distorted pentagonal bipyramidal complex, [MoO(O₂)(QO)₂], very efficiently catalyses homogeneous liquid phase oxidation of methylbenzenes, viz. toluene and o- and p-xylenes to benzoic acid, phthalic acid and p-toluic acid respectively, using H₂O₂ and O₂ as oxidants.

Full text of DOI:10.1039/a904440j

Novel oxo-peroxo molybdenum(vi) complexes incorporating 8-quinolinol:
synthesis, structure and catalytic uses in the environmentally benign and
cost-effective oxidation method of methyl benzenes: Ar(CH ) (n = 1, 2)
3
n
a
a
b
b
Ratna Bandyopadhyay, Sudeb Biswas, Subhadra Guha, Alok K. Mukherjee and Ramgopal
Bhattacharyya*
a
a
Department of Chemistry, Jadavpur University, Calcutta-700032, India
Department of Physics, Jadavpur University, Calcutta-700032, India
b
Received (in Cambridge, UK) 3rd June 1999, Accepted 14th July 1999
A hitherto unknown distorted pentagonal bipyramidal
complex, [MoO(O )(QO) ], very efficiently catalyses homo-
geneous liquid phase oxidation of methylbenzenes, viz.
toluene and o- and p-xylenes to benzoic acid, phthalic acid
and p-toluic acid respectively, using H O and O as
2 2 2
oxidants.
FeCl
3
solution, a deep green colour appears instantaneously due
]. Also, the {MoO(O } moiety is
2
2
to the formation of [Fe(QO)
3
2 2
)
6
formed readily. Hence, it appears that, albeit putatively, 1 is
correctly formulated. Interestingly, 1 reacts§ with toluene to
give benzoic acid (72%), with o-xylene to give phthalic acid
(55%) and with p-xylene to give p-toluic acid (61%) (all were
confirmed using the undepressed mixed melting point, and the
superimposability of the IR and NMR spectra with those of the
respective authentic acids as well as their methyl esters), with 2
produced also. The reaction with toluene is shown by eqn. (1) as
1
Oxo-peroxo molybdenum complexes catalytically oxidise a
2
variety of organic substrates, viz. alkenes, alcohols, amides and
nitro compounds etc., via homogeneous as well as heteroge-
3
neous routes. However, homogeneous liquid-phase hydro-
MoO(O
2
)
2
·2QOH + C
6 5 3 2
H –CH + O
carbon oxidation is rare.4 In this communication, besides
?
C
6
H
5
COOH + [MoO(O
2
)(QO)
2
] + 2H
2
O
(1)
reporting two new oxo-peroxo molybdenum(vi) compounds,
viz. MoO(O
2
)
2
·2QOH (1) and [MoO(O
2
)(QO)
2
] (2) (QOH =
a representative case. These observations led to the prediction
that 2 may be the catalyst precursor and 1 the active catalyst in
the oxidation of methylbenzenes. This has been excellently
verified by observing that 2 in a catalytic concentration
furnishes the same oxidations (Table 1) in refluxing acetonitrile
8-quinolinol),† and elucidating the structure of 2 by X-ray
crystallography,‡ we also report and substantiate that 2 is the
catalyst precursor and 1 the active catalyst in the homogeneous
and selective oxidation of toluene, o-xylene and p-xylene to
benzoic acid, phthalic acid and p-toluic acid respectively, under
very mild conditions, with an impressive yield and turnover
number and smooth catalyst recovery technique. This can be
regarded as one of the most inexpensive, environmentally
benign and industrially feasible methods of oxidising me-
thylbenzenes (which are very important by-products in coal-tar
distillation and petroleum cracking) to the corresponding
carboxylic acids, which have many uses in the chemical and
pharmaceutical industries.
in the presence of H
yield is reduced when only H
trace when exclusively O is used. The data in Table 1, suggest
that the catalytic process may be shown as Scheme 1. Table 1
shows that the use of an excess of H imparts a negative
effect on the catalytic oxidation, even in the double additive
case. The reason for this is at present obscure. When H is
used as the sole oxidant, it may undergo disproportionation
(H O + O ) under the catalytic influence of 2, and the O
2
O
2
and a brisk bubble of dioxygen/air. The
2 2
O
is used as the oxidant, and
2
2 2
O
2 2
O
2
2
2
The structure of 2 consists of discrete monomeric units of
produced is utilized to drive the reaction in a similar fashion as
in the twin reagent case. This hypothesis is supported by the fact
[
MoO(O
2
)(QO)
Cl
2
] (QO = 8-quinolinolate anion), and dis-
molecules held in the lattice. The geometry
ordered CH
2
2
that the process needs a much greater amount of H
necessary in the double additive method.
2 2
O than is
around the Mo atom can be best described as distorted
pentagonal bipyramidal (Fig. 1) with the axial sites being
occupied by the oxo (O5) and a phenolate oxygen (O1) ligands.
The other phenolate oxygen (O2), two N atoms (N1,N2) of the
2
2
bidentate QO ligands and the h -peroxo moiety (O3,O4)
define the equatorial plane with the Mo atom being displaced by
0
.175(1) Å from the equatorial plane towards the oxo oxygen.
The two essentially planar five-membered chelate rings (Mo,
O1, C8, C9, N1 and Mo, O2, C17, C18, N2) are approximately
orthogonal to each other,‡ the dihedral angle between the least
square planes through the ring atoms is 81.0(1)°. The bond
distance and angles (Fig. 1) correspond well to those of other
seven coordinate molybdenum oxo-peroxo complexes.5
Empirically 1 contains two atoms of oxygen and two atoms of
hydrogen more than 2. During crystallization or oxidation the
yellow solution of 1 changes to orange and displays a UV-Vis
spectrum which is identical to that of 2; 2 is actually deposited
from this orange solution. When in isolation 2 reacts with an
excess (Mo+H
observed under a UV-Vis probe). Analytical and infrared data
see Notes and references) indicate that 1 may be formulated as
MoO(O ·2QOH (1) or MoO(O )(QO) ·H (1a). However,
preliminary rate data show that the conversion of 1 to 2 at room
temperature is rather slow which is not compatible with its
formulation as 1a. Moreover, when 1 (but not 2) is treated with
2
O ≈ 1+4) of H
2 2
O to yield 1 (this is also
(
Fig. 1 An ORTEP view of [MoO(O
2 2
)(QO) ] 2 with selected bond distances
2
)
2
2
2
2 2
O
(Å) and angles (°): Mo–O1, 2.037(3); Mo–O3, 1.933(3); Mo–O5, 1.685(3);
Mo–N2, 2.269(3); Mo–O2, 1.993(3); Mo–O4, 1.925(3); Mo–N1, 1.441(4);
O3–Mo–O4, 43.86(13); O2–Mo–N2, 75.36(12); O1–Mo–N1, 74.87(11);
O1–Mo–O5, 156.25(14); O3–Mo–N2, 159.72(13).
Chem. Commun., 1999, 1627–1628
1627

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Table 1 Oxidation of methylbenzenes by H
O
2 2
and O
2
catalysed by [MoO(O
Oxidantc
2 2
)(QO)
]a
Substrateb
Time/h
6
Productd
Yield (%)
e
Turnover no.f
1310
C
6
H CH
5 3
(A)
H O
2 2
95
(0.038) + O
2
A
A
10
10
6
H
2
O
2
72
15
76
1019
21
(0.076)
D
D
2 2
H O
(0.190) + O
2
2
o-C
p-C
6
H
4
(CH
3
)
2
(B)
(C)
H
2
O
2
1047
(0.038) + O
B
10
6
H
2
O
2
55
88
757
(0.076)
E
6
H
4
(CH
3
)
2
H
2
O
2
1221
(0.038) + O
2
C
10
H O
2 2
60
832
(0.076)
F
a
25
2
is the catalyst precursor (1.15 3 10 moles used in each case), while 1 is the active species. No oxidation occurs without using 2 (or 1, though
b
much less efficiently). 0.016 mol used in each case. c Figures in parentheses indicate mol of H = one bubble per second. d See
footnote ¶. Based on mol of substrates used. Temp = reflux, solvent = acetonitrile. Is defined here as the ratio of the number of mol of product obtained
to the number of mol of catalyst used, in 1 batch.
O . Rate of flow of O
2 2 2
e
f
.73 g cm2 , m(Mo-Ka) = 0.990 mm21. 3359 observed [I > 2s(I)]
3
1
reflections. Enraf Nonius CAD4 diffractometer. Mo-Ka, R
0.1190 and GOF on F2 = 1.099. Structure solved and refined by
Patterson (SHELXS86) and successive Fourier and full matrix least squares
SHELXL93) methods. CCDC 182/1330. See http://www.rsc.org/suppdata/
cc/1999/1627/ for crystallographic files in .cif format.
In three different experiments 1 (0.05 g; 0.15 mmol) and (a) toluene (0.01
1 2
= 0.0414, wR
=
(
§
g; 0.11 mmol), (b) o-xylene (0.012 g; 0.11 mmol) and (c) p-xylene (0.012
g; 0.11 mmol) were separately dissolved in acetonitrile (15 ml), refluxed for
4
h and then cooled. In the case of toluene the resulting mixture was
evaporated to dryness, the mass extracted with diethyl ether and the extract
shaken with aqueous bicarbonate. On acidifying the aqueous layer, benzoic
acid was obtained. In the cases of o- and p-xylenes the respective products
Scheme 1
separated out on standing the solutions after CH
2 and (a) toluene, (b) o-xylene, (c) p-xylene were separately dissolved in
acetonitrile (15 ml) and 30% H (4 ml; 38 mmol) was added. The
3
CN reflux.
¶
2 2
O
It is extremely satisfying to note that the recovered catalyst in
the double additive process can be used again to oxidize a new
batch of the corresponding methylbenzenes showing similar
catalytic efficiency, but from the third batch onwards the
efficiency falls to a marked extent. Interestingly no oxidation
occurs if benzoquinone is added to the reaction medium, and a
resulting solution was refluxed for 6 h under bubbling dioxygen, cooled,
acetonitrile expelled, aqueous remains treated with diethyl ether and the
separated catalyst filtered from the aqueous solution. The ether extract was
treated with aqueous bicarbonate and the aqueous layer was acidified to
obtain benzoic acid. For phthalic acid (b) and p-toluic acid (c), after
3 2 2
evaporating off the CH CN the aqueous layer was subjected to CH Cl
extraction to separate the catalyst. The aqueous portion was then
concentrated using a rotary evaporator to get the respective acids. All the
acids obtained were chromatographically and analytically pure.
98% yield (vs. 30% without AIBN) is obtained after only 3 h
reflux when using AIBN [azobis(isobutyronitrile)], indicating
that the oxidation proceeds via a radical mechanism. All these
aspects will be detailed in due course.
Data collection for X-ray crystallography was done from
RSIC, Bose Institute, Calcutta. We thank the Alexander von
Humboldt-Foundation, Germany for the donation of the IR
spectrophotometer used and UGC and CSIR, New Delhi, for
financial support.
1
2
M. H. Dickman, Chem. Rev., 1994, 94, 569.
H. Mimoun, in Comprehensive Co-ordination Chemistry, ed. G.
Wilkinson, R. D. Gillard and J. A. McCleverty, Pergamon Press,
Oxford, 1987, vol. 6, and references therein; F. P Ballistreri, A. Bazzo,
G. A. Tomaselli and R. M. Toscano, J. Org. Chem., 1992, 57, 7074;
M. K. Trost and R. G. Bergman, Organometallics, 1991, 10, 1172.
3 K. T. Queeney, D. A.Chen and C. M. Friend, J. Am. Chem. Soc., 1997,
19, 6945 and references therein.
J. M. Aubry and S. Bouttemy, J. Am. Chem. Soc., 1997, 119, 5286.
1
4
Notes and references
5 N. M. Gresley, W. P. Griffith, B. C. A. Parkin, J. P. While and D. J.
Williams, J. Chem. Soc., Dalton Trans., 1996, 2039; P. K. Chakraborty,
S. Bhattacharya, C. G. Pierpont and R. Bhattacharyya, Inorg. Chem.,
†
3 2 2
1 was synthesised by dissolving MoO in 30% H O and adding an acetic
2
1
acid solution of QOH at 1+2 molar ratio. Yield 80%. IR (KBr) n/cm
:
1
992, 31, 3573.
7
(
MoNO) 960; (O–O) 860, 910(sh); (O–H) 3080 (b, H-bonded); Electronic
6
F. A. Cotton, G. Wilkinson, C. A. Murillo and M. Bochmann, Advanced
Inorganic Chemistry, 6th edn., John Wiley and Sons, New York, 1999,
p. 955.
2
2
spectrum of 1, l/nm: {QOH ? MoO(O
LMCT} 375 (sh). 2 was obtained by slow crystallization of 1 from CH
2
)
2
CT} 355, {O
2
? Mo(vi)
Cl
2
2
–
-1
hexane or CH
9
3 2
CN–Et O, yield 60%. IR (KBr) n/cm : (MoNO) 945; (O–O)
7
8
9
S. E. Jacobson, R. Mares and F.Mares, Inorg. Chem., 1978, 17, 3061;
M. T. H. Tarafder and N. S. Islam, Polyhedron, 1992, 11, 795.
P. C. H. Mitchell, Quart. Rev. (London) 1966, 20, 103; R Bhattacharyya
and S. Ghosh, Indian. J. Chem. A, 1991, 30, 35.
2
8
2
20; vibrations of QO comparable to those of [MoO (QO) ] where QO
2 2
9
2
10
is bidentate; electronic spectrum, l/nm: {QO ? Mo(vi) LMCT} 400;
2
2
{
O
2
? Mo(vi) LMCT} 375 (sh). Satisfactory elemental analyses
(C,H,N,Mo) were obtained for all the isolated complexes.
T. Venkataraman and K. S. Nagaraja, Polyhedron, 1992, 2, 185.
‡
Crystal data for 2: [MoO(O
2
)(QO)
2
], C19
orange, triclinic, space group P1, a = 9.791(1) Å, b = 10.643(1) Å, c =
0.964(2) Å, a = 117.70(2)°, b = 90.14(1)°, g = 105.08(1)°, V = 966.5
2 2 5
H14Cl N O Mo, M = 517.16,
1
0 C. Djordjevic, B. C. Puryear, N. Vuletic, C. J. Abett and S. J. Sheffield,
¯
Inorg. Chem., 1988, 27, 2926.
1
3
23
(3) Å , T = 293(2) K, Z = 2, l = 0.71093 Å, D
c
= 1.777 g cm
D
m
=
Communication 9/04440J
1628
Chem. Commun., 1999, 1627–1628