Oxoperoxo molybdenum(VI) and tungsten(VI) and oxodiperoxo molybdate(VI) and tungstate(VI) complexes with 8-quinolinol: Synthesis, structure and catalytic activity

Article abstract of DOI:10.1039/b410984h

A solution obtained by dissolving MoO₃ in a moderate excess of H₂O₂ reacts with 8-quinolinol (QOH) to give [MoO(O ₂)(QO)₂] (1), but, when the same reaction is conducted with a large excess of H₂O₂, an anionic complex is formed, which reacts with PPh₄Cl to give the corresponding salt [MoO(O ₂)₂(QO)][PPh₄] (2 · PPh₄). Freshly prepared WO₃ behaves the same way and, depending on the amount of H₂O₂ used, as above, produces either [WO(O ₂)(QO)₂] (3) or [WO(O₂)₂(QO)] [PPh₄] (4 · PPh₄), respectively. Crystallographic analyses reveal the coordination geometries around the metal center in these complexes to be distorted pentagonal bipyramids. These compounds show interesting catalytic properties in the oxidation of alcohols using H ₂O₂ as the terminal oxidant. In the case of aromatics, including benzylic and cinnamylic alcohols, the oxidation occurs selectively, affording aldehydes or ketones with reasonably high turnover numbers. Taking benzyl alcohol as a representative case, a probable mechanism of the alcohol-to-aldehyde conversion mediated by the prepared catalysts is suggested. The oxidation of aliphatic primary alcohols, under the same conditions, does not show the above selectivity: the reaction yields the corresponding aldehydes as well as carboxylic acids. The work was also extended to study the catalytic activity towards the oxidation of phenol and various sulfides and amines using the same oxidants. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2005.

Full text of DOI:10.1039/b410984h

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P A P E R
Oxoperoxo molybdenum(VI) and tungsten(VI) and oxodiperoxo
molybdate(VI) and tungstate(VI) complexes with 8-quinolinol:
synthesis, structure and catalytic activity
Swarup K. Maiti,a Surajit Banerjee,b Alok K. Mukherjee,b K. M. Abdul Malikc and
Ramgopal Bhattacharyya*a
a Department of Chemistry, Jadavpur University, Calcutta, 700032, India.
E-mail: aargibhatta@yahoo.com; Fax: +91 33 2414-6584; Tel: +91 33 2414-6193;
b Department of Physics, Jadavpur University, Calcutta, 700032, India
c Department of Chemistry, Cardiff University, PO Box 912, Park Place, Cardiff,
UK CF10 3TB
Received (in Montpellier, France) 17th July 2004, Accepted 11th November 2004
First published as an Advance Article on the web 18th February 2005
A solution obtained by dissolving MoO3 in a moderate excess of H2O2 reacts with 8-quinolinol (QOH) to
give [MoO(O2)(QO)2] (1), but, when the same reaction is conducted with a large excess of H2O2, an anionic
complex is formed, which reacts with PPh4Cl to give the corresponding salt [MoO(O2)2(QO)][PPh4]
(2 ꢀ PPh4). Freshly prepared WO3 behaves the same way and, depending on the amount of H2O2 used,
as above, produces either [WO(O2)(QO)2] (3) or [WO(O2)2(QO)][PPh4] (4 ꢀ PPh4), respectively.
Crystallographic analyses reveal the coordination geometries around the metal center in these complexes to
be distorted pentagonal bipyramids. These compounds show interesting catalytic properties in the oxidation
of alcohols using H2O2 as the terminal oxidant. In the case of aromatics, including benzylic and cinnamylic
alcohols, the oxidation occurs selectively, affording aldehydes or ketones with reasonably high turnover
numbers. Taking benzyl alcohol as a representative case, a probable mechanism of the alcohol-to-aldehyde
conversion mediated by the prepared catalysts is suggested. The oxidation of aliphatic primary alcohols,
under the same conditions, does not show the above selectivity: the reaction yields the corresponding
aldehydes as well as carboxylic acids. The work was also extended to study the catalytic activity towards the
oxidation of phenol and various sulfides and amines using the same oxidants.
were found to be stable at ambient temperature and behaved as
Introduction
stoichiometric reagents for substrate oxidation, themselves
being converted to their respective monoperoxo species.14
However, the catalytic activity of those compounds have not
yet been examined.15
Molybdenum and tungsten, the 4d and 5d congeners of group
6 metals, are long known to function as potential heteroge-
neous catalysts for the oxidation of organic compounds, when
in their higher oxidation states.1 Homogeneous oxo-transfer
chemistry of the MO22+ (M = Mo, W) core is known to be
biomimetic.2–4 Indeed, peroxo molybdenum and tungsten sys-
tems have been recognized as potential insulin mimics5 and
some oxo and oxoperoxo molybdenum and tungsten species
have recently been shown to possess superior bromoperoxidase
activity to that of peroxo vanadates.6
If much work has been done on modelling studies for Mo,7–10
oxo-transfer modelling involving W is much less explored. The
first work was reported by Holm and Yu.11 The presence of a
cis-[WO2S4] group was established in an active site of ferre-
doxin aldehyde oxidoreductase.12 In oxoperoxo chemistry of
Mo and W an important structural motif, in which two peroxo
groups and a doubly bonded oxo ligand create the median
M(O2)2O plane, is well known.13 This core, although observed
as being most stable13 and a common motif in oxoperoxo
molybdenum systems, has, in our experience, a high formation
tendency without doubt but is also a rather reactive species,
which readily performs substrate oxidation, converting itself
into a MO(O2)2+ core, which gives more stable compounds
than its diperoxo analogue. A group of compounds containing
an MoO(O2)2 core with non-deprotonated a-amino acid li-
gands and of general composition [MoO(O2)2(L-L0H)], where
L-L0H = glycine, alanine, proline, valine, leucine or serine,
Complexes of the type [MoO(O2)2L]16 where L = (Me2N)3-
PO, R3PO (R = alkyl or aryl), DMF, DMSO or pyridine, are
known to be useful catalysts, mainly in the epoxidation of
olefins.17 Such epoxidation reactions were studied by Venture-
llo et al.18 and Ishii et al.,19 using molybdenum complexes as
catalysts during their work on alcohol oxidation. Notably,
Modena and co-workers20 reported a wide variety of stoichio-
metric20a as well as catalytic20b alcohol-to-aldehyde oxidation
reactions by using Mo- and W-based oxoperoxo complexes.
Interestingly, [MoVIO2 (acac)2] (acac = acetyl acetonate) was
shown11 to function as a catalyst in the homogeneous oxida-
tion of alcohols to aldehydes or ketones by O2, but the
method is a bit cumbersome, requiring copper salts as co-
catalyst and addition of toluene in the reaction mixture.21
Trost and Masuyama22 reported heptamolybdate-catalyzed
alcohol oxidation by H2O2, but with very low turnover num-
bers (TON).
In a previous communication,23 we have shown for the first
time that an oxomonoperoxo molybdenum(VI) complex,
[MoO(O2)(QO)2] 1, with QOꢁ = 8-quinolinolate, is able to
catalyze (very efficiently) the oxidation of alkyl benzenes. 1
catalyzes the said oxidation using a moderate excess of H2O2 as
oxidant, while large excesses lower the TON. Strangely en-
ough, we found that its tungsten analogue [WO(O2)(QO)2] 3,
T h i s j o u r n a l i s & T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 5
554
N e w J . C h e m . , 2 0 0 5 , 2 9 , 5 5 4 – 5 6 3

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which we reported very recently,24 fails to catalytically oxidize
alkyl benzenes, either with moderate or excessive amounts of
H2O2. To rationalize the diminished catalytic ability of 1 in the
presence of excess of H2O2, we have recently discovered that
QOH is capable of coordinating to the MO(O2)2 core when the
putative [MO(O2)2 ꢀ 2QOH] (M = Mo,23 W24) adduct is dis-
solved in an excess of H2O2, to afford monoanionic diperoxo
complexes [MO(O2)2(QO)]ꢁ [M = Mo (2) and W (4)], which
could be isolated as PPh4 salts.
In the present work, we report the detailed synthesis, struc-
ture and catalytic activity of 1-4, where it will be apparent that
the anionic complexes have lower catalytic capabilities than
their neutral counterparts. Although 2 and 4 are inactive in the
catalytic oxidation of alkyl benzenes, the key result of the
present work is the selective conversion of benzyl and cinnamyl
alcohol into benzaldehyde and cinnamaldehyde, that is, with-
out further oxidation to their corresponding carboxylic acids.
However, aliphatic alcohols did not show the above selectivity.
The work is extended to other important substrates, such as
phenol, various sulfides and amines.
Syntheses
[MoO(O2)(QO)2] (1). MoO3 ꢀ 2H2O (1.25 g, 6.95 mmol) was
dissolved in 30 wt % H2O2 (40 ml, 13.9 mmol) by stirring at
room temperature, giving a pale yellow solution. Addition of 8-
quinolinol (2.02 g, 13.9 mmol) as a dilute (4 M) acetic acid
solution (10 ml) to the above mixture, under stirring, gave 5
[MoO(O2)2 ꢀ 2QOH] as a yellow solid.21 The solid was filtered
off, washed thoroughly with water, 95% ethanol and diethyl
ether, then dried under vacuum. Yield of 5 was 2.76 g (86%).
Slow crystallization from dichloromethane–hexane (5 : 1) gave
pure 1 as shiny orange crystals (crystallization yield: 95%). 1
was soluble in dichloromethane, acetonitrile and acetone but
insoluble in ethanol, benzene and diethyl ether. Anal. calcd for
C19H14Cl2N2O5Mo: C, 44.1; H, 2.7; N, 5.4; Mo, 18.6; found:
C, 49.8; H, 2.7; N, 6.3; Mo, 23.1; IR (cmꢁ1): 1620 (w), 1525 (s),
1480 (s), 1400 (s), 1325 (s), 1250 (m), 1220 (m), 1200 (w), 1115
(s), 960 (s, n Mo O), 915 (m, nO–O), 850 (s), 800 (w), 790 (m),
740 (s), 660 (s), 590 (s), 545 (m), 520 (w), 480 (w), 315 (m); UV-
vis (lmax/nm): 364 (e = 4720 Mꢁ1 cmꢁ1).
Q
.
[MoO(O2)2QO][PPh4] (2 PPh4). 5 (1.17 g, 2.50 mmol) was
Experimental
dissolved in the minimum volume of acetonitrile (20 ml) in a
conical flask fitted with an air condenser and was stirred for 1 h
after the addition of an excess of H2O2 (30 wt %, 15 ml); a clear
yellow solution was obtained. Dropwise addition of an aqu-
eous solution (10 ml) of PPh4Cl (0.86 g, 2.5 mmol), under
constant stirring for another 30 min., gave crude 2 ꢀ PPh4 as a
shiny light yellow solid. The compound was found to be
soluble in acetonitrile, acetone, dichloromethane and chloro-
form but insoluble in ethanol and diethyl ether. It was crystal-
lized from dichloromethane–hexane (1 : 1) to provide pure 2 as
rectangular light yellow crystals. Yield: 1.25 g (76%); anal.
calcd for C33H26O6NPMo: C, 60.0; H, 4.0; N, 2.1; Mo, 14.6;
found, C, 60.2; H, 4.0; N, 2.2; Mo, 14.4; IR (cmꢁ1): 3040 (w),
1610 (w), 1600 (w), 1580 (m), 1560 (m), 1490 (s), 1460 (vs), 1455
(s), 1450 (sh), 1375 (s), 1320 (s), 1280(s), 1100 (vs), 1020 (sh),
Materials
The chemicals MoO3 ꢀ 2H2O, Na2WO4 ꢀ 2H2O, dinitrophenyl-
hydrazine and 8-quinolinol were of extra pure quality and
obtained from Loba Chemie (India). Hydrogen peroxide
(30%), cyclohexanol, 1,4-diaminobenzene, acetonitrile, di-
chloromethane, light petroleum (40–60), diethyl ether and
acetone were of analytical grade and were obtained from
Merck (India). Tetraphenylphosphonium chloride (extra pure
quality) and AnalaR grade dimethyl sulfide were obtained
from Merck (Germany). Acetonitrile, dichloromethane and
acetone were further purified before use following literature
methods.25 Sodium bicarbonate, triphenylphosphine, benzyl
alcohol and phenol were from Sisco Research Laboratories
(SRL, India) and were used as received. Acids and alkalis were
of AR grade from Merck (India). Ethanol (90%) was obtained
from Bengal Chemical and Pharmaceutical works (Calcutta)
and was distilled over lime before use. Ultra high pure (UHP)
grade dioxygen, dinitrogen, zero CO2 air and dihydrogen gases
were used whenever necessary, including in chromatographic
analysis. HPLC or GR grade solvents (Merck, Germany) were
used for quantitative gas chromatographic (GC) analysis, with
di-n-butyl ether as internal standard.
1000 (m), 945 (s, nMo O), 845, 810 (m, nO–O), 800 (w), 780 (m),
Q
760 (s), 740 (s), 720 (s), 645 (s), 580 (s), 530 (s), 460 (w), 380 (w),
310 (m); UV-vis (lmax/nm): 357 (e = 3725 Mꢁ1 cmꢁ1).
[WO(O2)(QO)2)] (3). An aqueous solution (25 ml) of
Na2WO4 ꢀ 2H2O (1.98 g, 6.0 mmol) was acidified with a 6 M
HCl solution until a white precipitate of WO3 ꢀ nH2O was
obtained. It was filtered off and washed several times with
water and ethanol. The precipitate was transferred quantita-
tively into a beaker and was dissolved in 30 wt % H2O2 (10 ml,
13.9 mmol) by stirring at room temperature (25 1C) until a
clear and colourless solution was obtained. Addition of 8-
quinolinol (1.74 g, 12.0 mmol) dissolved in acetic acid (6 M, 10
ml) to the clear solution while stirring (about 10 min) gave 6
[WO(O2)2 ꢀ 2QOH] as a yellow solid, which was filtered off,
washed with water, 95% ethanol and diethyl ether. Yield of 6
was 2.73 g (ca. 82%). A fraction of 6 (1.39 g, 2.5 mmol) was
dissolved in acetonitrile (20 ml) and refluxed for 20–25 min.
The solution was then cooled and diethyl ether was added to
the cold solution until the solvent mixture was 2 : 1 (v/v) in
acetonitrile and diethyl ether, respectively. Upon standing for
15 h, this solution afforded pure 3 as orange crystals (crystal-
lization yield: 1.2 g, 92%). Compound 3 was soluble in
acetonitrile, dichloromethane, acetone and chloroform, but
insoluble in diethyl ether, benzene and ethanol. Anal. calcd
for C18H12N2O5W: C, 41.5; H, 2.3: N, 5.4; W, 35.4; found, C,
42.1; H, 2.4; N, 5.6; W, 35.2; IR (cmꢁ1): 1590 (w), 1510 (s),
1470 (s), 1460 (sh), 1380 (s), 1330 (s), 1270 (m), 1240 (w), 1110
(s), 960 (s, nWQO), 890 (m, nO–O), 825 (m), 790 (w), 780 (sh), 755
(s), 630 (m), 530 (m), 500 (m); UV-vis (lmax/nm): 367 (e = 2870
Mꢁ1 cmꢁ1).
Physical measurements
IR spectra were recorded as KBr pellets on a Perkin Elmer
597 IR spectrophotometer (4000–200 cmꢁ1) and electronic
spectra on a Hitachi U-3410 UV-vis NIR spectrophotometer.
A Systronics (India) model 335 digital conductivity bridge
with a bottle-type cell was used to measure the solution
conductance values of the isolated complexes at 25 1C using
a thermostatic arrangement. A SUVNIC (UK) apparatus
was used to measure melting points of organic substrates as
well as their oxidation products. The magnetic susceptibilities
were obtained by the Guoy method using Hg[Co(NCS)4]
as calibre. Elemental analysis (C, H and N) was performed
with a Perkin Elmer 240C elemental analyzer. W and Mo were
estimated gravimetrically as WO3 and [MoO2(QO)2], res-
pectively.26 Triply distilled (all glass) water was used through-
out. GC measurements were routinely done on a HP 5880A
gas chromatograph using an OV-101 packed column. How-
ever, the data presented here were recorded with an Agilant
model 6890 N gas chromatograph using HP-1 and INNO-
WAX capillary column in FID mode with dinitrogen as
carrier gas.
N e w J . C h e m . , 2 0 0 5 , 2 9 , 5 5 4 – 5 6 3
555

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.
[WO(O2)2QO][PPh4] (4 PPh4). 6 (1.39 g, 2.5 mmol) was
taken out for GC analysis. For time versus yield studies,
aliquots (1 ml) were injected into the GC equipment at 1 h
intervals. For the isolation and quantitative estimation of the
products, the solvent (CH3CN) was distilled out and the
residual liquid (H2O from H2O2) was shaken with CH2Cl2
(5 ml) in a decanting flask in which aqueous and organic layer
separated. The latter was isolated and the aqueous layer was
repeatedly (3–4 times) washed with CH2Cl2. The organic
extracts were combined and the solvent was distilled out. The
residue was extracted with ether, from which the oxidation
products were isolated and characterized, according to proce-
dures I–III described below. The insoluble residue left after
ether extraction looked like the catalyst, which was confirmed
by IR spectroscopy. In all cases the identity and the concen-
tration of the oxidation products were confirmed by gas
chromatography. It should be mentioned that for all the
procedures below the amount of products chemically separated
corresponded closely to the GC results. Work-up procedure I
was used to isolate aldehydes or ketones and procedures II and
III were used to isolate carboxylic acids and other compounds.
dissolved in the minimum volume (15 ml) of acetonitrile to
which an excess (15 ml) of H2O2 (30 wt %) was added and the
resulting solution was stirred for ca. 90 min, when the colour of
the solution assumed a light yellow shade. Dropwise addition
of PPh4Cl (0.86 g, 2.50 mmol) dissolved in water (20 ml) to the
above solution while stirring gave crude 4 ꢀ PPh4 as a light
yellow solid. Its solubility was the same as that of 1 and 2 ꢀ PPh4
and the compound was crystallized as yellow rectangular plates
from dichloromethane–n-hexane (1 : 1). Yield: 1.38 g (ca.
74%); anal. calcd for C33H26O6NPW: C, 53.0; H, 3.2; N, 1.9;
W, 24.6; found, C, 53.3; H, 3.7; N, 2.1; W, 24.4; IR (KBr disc,
cmꢁ1): 1600 (w), 1580 (m), 1500 (s), 1480 (w), 1460 (s), 1440 (s),
1430 (sh), 1375 (s), 1325 (s), 1260 (m), 1190 (w), 1160 (w), 1105
(vs), 1025 (w), 1000 (m), 945 (s, n WQO), 830 (sh), 825 (m), 820
(sh, italicized vibrations, nO–O), 800 (w), 780 (m), 760 (m), 750
(sh), 720 (s), 690 (s), 625 (s), 575 (m), 525 (vs), 450 (w), 425 (w),
385 (w), 300 (m), 290 (sh); UV-vis (lmax/nm): 395 (sh, 30), 336
(e = 1510 Mꢁ1 cmꢁ1).
X-Ray crystallographic data collection and refinement
Procedure I. Carbonyl compounds were isolated as yellow
orange solids in the form of their 2,4-dinitrophenyl hydrazone
derivatives, the corresponding carbonyls being generated by
acid hydrolysis. The purity of the 2,4-dinitrophenyl hydrazone
derivatives was checked by integration of the 1H NMR spectra.
X-Ray intensity data for complexes [MoO(O2)(C9H6ON)2] ꢀ
CH2Cl2 (1 ꢀ CH2Cl2), [WO(O2)(C9H6ON)2] (3), [MO(O2)2
(C9H6ON)2][PPh4], M = Mo (2 ꢀ PPh4) or W (4 ꢀ PPh4), were
collected at 20 1C (ꢁ123 1C for 3) with an Enraf–Nonius CAD
4 diffractometer (o scan) using graphite-monochromated
MoKa radiation (l = 0.71073 A). Unit cell parameters were
determined by least-squares refinements of setting angles for 25
reflections with 8 o y o 181. Data were corrected for Lorentz
polarization effects. Empirical absorption correction using c
scan27 (DIFABS28 for 3) were applied. The crystal structures
were solved by the heavy atom method using SHELXS-8629
and refined on F2 by SHELXS-9730 with anisotropic thermal
parameters for nonhydrogen atoms (disordered solvent
CH2Cl2 atoms in the solvate of 1 were treated isotropically).
The oxo group in 3 was disordered over two sites with
occupancy factors of 0.5. The hydrogen atoms placed in the
calculated positions were allowed to ride on the atoms to which
they were attached with the thermal parameters tied to those of
the parent atoms.w
Procedure II. The reaction solutions presumed to contain
carboxylic acids were treated with aqueous NaHCO3; the
aqueous layers were isolated and concentrated almost to
dryness and allowed to stand for 30 min. The acids were
isolated as their corresponding colourless sodium salts.
Procedure III. The sulfones were crystallized out as solids by
concentrating the aqueous layer, while unreacted sulfides and
sulfoxides, remaining in CH2Cl2, were separated by fractional
distillation. 1,4-Benzoquinone (from phenol) was extracted
from the reaction solution by diethyl ether; evaporation of
ether deposited the off-white material. The mixture of pro-
ducts, 1,4-benzoquinone and 4-nitroaniline, obtained from 1,4-
diaminobenzene was separated by steam distillation since the
former is steam-volatile. Other amines and their oxidation
products were separated by column chromatography and their
identities were confirmed by NMR spectroscopy.
Catalytic oxidation studies
All complexes exhibited general catalytic oxidizing activity
towards alcoholic, sulfide and amine functions, converting
organic substrates into their oxidized products with impressive
yields as well as high turnover numbers (TON). The following
equation is an example of the above mentioned catalytic
reactions:
Results and discussion
Synthetic aspects and general characterization
½MOðO2Þ2ðQOÞꢂꢁ M ¼Mo;Wð0:1 mol % Þ
Alcohols
ꢁ! ð4 equiv:; 400 mol % Þ Aldehydes=Ketones
1 and 3 were prepared by the same method, the former by using
MoO3 and the latter, freshly prepared WO3 as starting materi-
als. The putative compounds23 [MoO(O2)2 ꢀ 2QOH] (5) and
[WO(O2)2 ꢀ 2QOH] (6) were formed first, and crystallization
from suitable solvents afforded 1 and 3, respectively. The
intermediates 5 and 6 were separately dissolved in excess
H2O2 and stirred, thus forming the anionic species
[MO(O2)2QO]ꢁ [M = Mo (2), W (4)]; the addition of PPh4Cl
allowed the precipitation of their respective PPh4 salts.
While 1 and 3 are non-electrolytes, 2 ꢀ PPh4 (125 ohmꢁ1 cm2
molꢁ1) and 4 ꢀ PPh4 (120 ohmꢁ1 cm2 molꢁ1) behave, as ex-
pected, as 1 : 1 electrolytes in acetonitrile.31 The QOꢁ - M(VI)
LMCT in [MoO(O2)(QO)2] (1) occurs at a lower wavelength
(364 nm) than that in [WO(O2)(QO)2] (3), where it occurs at
367 nm (see also the Experimental). The low-energy shift in the
electronic transition on going from molybdenum to tungsten is
more marked in the two anionic complexes: the QOꢁ - M(VI)
LMCT transition in 2 ꢀ PPh4 appears at 357 nm, while for 4 ꢀ
PPh4, the same occurs at 395 nm. This observed wavelength
CH3CN or CH2Cl2; H2O2
ðiÞ
Alcohols and other substrates (25.0 mmol) were weighed
directly into 50 ml two-necked round-bottomed flasks and
dissolved in 10 ml CH3CN or 15 ml CH2Cl2; catalysts 2 or 4
were added to each of the above solutions as their PPh4 salts
(0.025 mmol, 0.1 mol %). The solutions were then treated with
30% H2O2, added in portions, for a total of 100 mmol (400 mol
%, 4 equiv. with respect to the substrate and 4000 equiv. with
respect to the catalysts); the resulting solutions were brought to
reflux (78 1C for acetonitrile and 40 1C for CH2Cl2). Dioxygen
was bubbled through the solutions at a rate of 4 bubbles per
second when effect of dual additions was investigated. The
reaction mixtures were then cooled to 25 1C and aliquots were
w CCDC reference numbers [CCDC NUMBER(S)]. See http://
www.rsc.org/suppdata/nj/b4/b410984h/ for crystallographic data in
.cif or other electronic format.
556
N e w J . C h e m . , 2 0 0 5 , 2 9 , 5 5 4 – 5 6 3

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shift from Mo to W is due to the lower optical electronegativity
of W(VI) compared to that of Mo(VI).
The n(M O) vibration appears as strong bands at 960, 945,
Q
945 and 960 cmꢁ1 in 1, 2 ꢀ PPh4, 3 and 4 ꢀ PPh4, respectively.
The corresponding n(O–O) vibration appears as medium in-
tensity bands at 915 (1), 845 and 810 (2 ꢀ PPh4), 890 (3), and
830, 825 and 820 (4 ꢀ PPh4) cmꢁ1, indicating that in 2 ꢀ PPh4 and
4 ꢀ PPh4 the oxodiperoxo core does not assume a strictly
median plane with respect to the entire molecule. This is also
evident from their molecular structures (Fig. 1 and Fig. 2). The
n(M–N) and n(M–O) from the quinolinol ligand complexation
may be assigned to the weak bands around 450 and 425 cmꢁ1
,
respectively, in all cases. A medium-to-strong intensity band in
the 590 cmꢁ1 region and another at 530 cmꢁ1 are assignable to
the asymmetric and symmetric vibrations of the MO2 triangle,
respectively.32 In the case of 2 ꢀ PPh4 and 4 ꢀ PPh4, the sym-
metric vibration is hidden by the very strong Ph4P+ vibration
in that region. Other major vibrations originate from the 8-
quinolinol ligands, modified as typical in metal-coordinated
bidentate species.33
Molecular structures
Fig. 2 Molecular structure of the anion [WO(O2)2(QO)]ꢁ, 4, showing
the atom numbering scheme. Hydrogen atoms are omitted for clarity.
Crystal data and refinement results for the studied compounds
are given in Table 1. The crystal structure determinations of
[MO(O2)(QO)2] [M = Mo (1), W (3)] have been reported in
earlier communications.23,24 The coordination geometry
around the metal center in 1 and 3 can be described as distorted
pentagonal bipyramids with the two essentially planar QOꢁ
ligands being approximately orthogonal to each other; the
dihedral angles between the least-square planes through the
ligand atoms in 1 and 3 are 84.6(1)1 and 83.1(1)1, respectively.
The structures of 2 ꢀ PPh4 and 4 ꢀ PPh4 consist in discrete
monomeric anions, [MO(O2)2(QO)]ꢁ [M = Mo (2) , W (4)],
and PPh4+ cations held together in the lattice. Both PPh4 salts
are characterized by an MO4+ center with the oxo groups
occupying the apical positions. The geometry about the metal
center in 2 ꢀ PPh4 and 4 ꢀ PPh4 can be best described as penta-
gonal bipyramids (Fig. 1 and Fig. 2) with the axial sites
occupied by the oxo (O6) and the quinoline (N1) ligands.
Two Z2-peroxo moieties (O2, O3 and O4, O5) and the phenol
oxygen O1 from the quinoline ligand define the equatorial
planes in 2 and 4 with the metal atoms being displaced towards
the oxo oxygen by 0.340(1) A in 2 and 0.345(1) A in 4, from the
corresponding equatorial plane. This is in agreement with the
observation from various structurally characterized oxodiper-
oxo complexes of molybdenum(VI) and tungsten(VI),34 in which
a high stability of the oxodiperoxo molybdate and tungstate
complexes is attained when the two peroxide groups coordinate
in the equatorial plane. The essentially planar 8-quinolinolate
ligand (N1, C1–C9) is almost orthogonal to the equatorial
plane (O1–O5) in both complexes, the dihedral angles between
the two least-squares planes in 2 and 4 being 92.2(2)1 and
91.6(2)1, respectively. Selected bond distances and angles for 2
and 4 (Table 2) correspond well to those of other seven-
coordinate Mo and W oxoperoxo complexes.23,35 The length-
ening of the Mo–N [2.395(2) A] and W–N [2.364(10) A]
distances in 2 and 4 compared to the Mo–N [2.194(3)–
2.269(3) A] and W–N [2.264(6)–2.273(6) A] bond lengths in 1
and 3 reflects the strong trans influence of the oxo ligand.36
Catalytic properties of the isolated complexes
All complexes possess general catalytic and selective oxidizing
properties towards alcoholic functions from organic substrates
(Table 3), such as benzyl and cinnamyl alcohol selectivity to
benzaldehyde and cinnamaldehyde or isopropanol and cyclo-
hexanol to acetone and cyclohexanone, respectively, with im-
pressive yields. However, this selectivity is not observed when
the substrates are aliphatic alcohols, since a mixture of the
Fig. 1 Molecular structure of the anion [MoO(O2)2(QO)]ꢁ, 2, show-
ing the atom numbering scheme. Hydrogen atoms are omitted for
clarity.
N e w J . C h e m . , 2 0 0 5 , 2 9 , 5 5 4 – 5 6 3
557

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Table 1 Crystal and refinement data for the studied compounds
1 ꢀ CH2Cl2
2 ꢀ PPh4
3
4 ꢀ PPh4
Formula
Formula weight
C19H14Cl2N2O5Mo
517.16
Triclinic
C33H26NO6PMo
659.46
Triclinic
C18H12N2O5W
520.15
Triclinic
C33H26NO6PW
747.37
Triclinic
Crystal system
Space group
ꢀ
P1
ꢀ
P1
ꢀ
P1
ꢀ
P1
a/A
b/A
9.791(1)
10.643(1)
10.964(2)
117.70(2)
90.14(1)
105.08(1)
966.5(3)
2
9.928(1)
11.221(1)
13.236(1)
86.48(1)
78.97(1)
78.57(1)
1418.2(3)
2
7.930(2)
8.221(2)
12.625(2)
89.71(1)
78.99(1)
82.00(1)
799.9(3)
2
9.883(3)
11.205(5)
13.138(2)
86.25(7)
79.40(5)
78.52(3)
1400.9(19)
2
c/A
a/1
b/1
g/1
U/A3
Z
Dcalcd (Do)/mg mꢁ3
F(000)
1.770 (1.720)
516
0.990
1.544 (1.560)
672
0.567
2.160 (2.130)
496
7.255
1.772 (1.760)
736
4.229
m/mmꢁ1
Reflections collected
3359
3006
5411
4175
3193
2565
3654
2820
Observed reflections [I 4 2s(I)]
R1
wR2
0.041
0.1190
0.030
0.077
0.034
0.087
0.047
0.119
respective aldehydes and carboxylic acids are obtained. Lack of
selectivity in the case of aliphatic alcohols is due to the fact that
As shown in Table 3, the tungsten anion 4 is a more efficient
catalyst than the analogous molybdenum anion 2. Tungsten
complex 3 is also superior24 to the corresponding molybdenum
complex 1. Plots of yield percentage versus time for the
oxidation of some representative substrates, that is, cyclic
aliphatic (entry 1), aromatic (entry 2), olefinic aromatic (entry
3), and primary aliphatic (entry 5) alcohols are presented in
Fig. 3 and those of a secondary alcohol (entry 11), a phenol
(entry 13) and an aromatic amine (entry 19) are shown in
Fig. 4, with 4 as a representative catalyst and H2O2 as the
oxidant.
resonance stabilization of the C O bond in aromatics makes
Q
it reasonably immune against a nucleophilic attack37,38 (here
by O22ꢁ). Actually, as long as there is a noticeable concentra-
tion of H2O2, the aromatic aldehydes are not oxidized to their
corresponding acids. It is observed that, when increasing length
in aliphatic alcohols, the rate of catalytic oxidation decreases
and, for instance, the corresponding aldehyde can be obtained
exclusively from n-dodecanol (entry 12). We have singled out
the CH3OH-to-HCHO oxidation scheme for its industrial and
pharmacological significance and work is ongoing in this area
to make it a selective reaction using additives and to improve
yields.
The complexes [MO(O2)(QO)2] (M = Mo, W) stoichiome-
trically oxidize benzyl alcohol almost quantitatively to benzal-
dehyde; both charge and mass balances are shown in eqns.
(1)–(3).
Notably, oxidation of 1,4-diaminobenzene (entry 17) to
quinone is an important industrial process, though the simul-
taneous production of quinone along with p-nitroaniline is
interesting from the reactivity viewpoint. A reduction in reflux-
ing time increases the proportion of p-nitroaniline produced;
the same occurs with aniline (entry 19), with an increase of
nitrosobenzene production. In the case of diaminocyclohexane
(entry 18), the amount of corresponding oxidized product,
dioxime, increases when increasing reflux time. The oxidation
of dimethylsulfide (entry 14) to dimethylsulfone goes via a
dimethylsulfoxide intermediate. If the time of reflux and the
amount of peroxide are reduced, it is possible to get a selective
and quantitative yield of DMSO and the same observations
were made for the other two sulfides (entries 15 and 16).
[MO(O2)(QO)2] + 2eꢁ - [M(O)2(QO)2] + O2ꢁ
C6H5CH2OH - C6H5CHO + 2H+ + 2eꢁ
Adding eqns. (1) and (2) gives eqn. (3):
(1)
(2)
[MO(O2)(QO)2] + C6H5CH2OH -
[M(O)2(QO)2] + C6H5CHO + H2O
(3)
It has been experimentally verified by us that [M(O)2(QO)2]
is the end product of this stoichiometric reaction [eqn. (3)]
and the said dioxo species, upon H2O2 treatment produce
[MO(O2)(QO)2], which again oxidize benzyl alcohol to
benzaldehyde. So, one may presume that [M(O)2(QO)2] is a
catalyst precursor and [MO(O2)(QO)2] the active catalyst.
Still, we have shown23 that, when H2O2 is the oxidant, it reacts
with [MO(O2)(QO)2] to produce either [MO(O2)2 ꢀ 2QOH]
in the presence of a moderate excess of H2O2 or the anionic
complex [MO(O2)2(QO)]ꢁ in the presence of a large excess of
H2O2, as shown in the present work where this anionic
complex has been isolated as its PPh4 salt and structurally
characterized. Hence, it is quite safe to assume that the
QOH adducts22 (5 and 6, see Experimental) are the active
catalysts in a reaction media with a moderate excess of
H2O2 and the anionic complexes are the active catalysts
when large excesses of H2O2 are used. With these assumptions,
we suggest the probable reaction routes for catalytic oxidation
reactions with [MO(O2)(QO)2] as catalyst precursor and
H2O2 as oxidant, both in the presence of a moderate excess
of the oxidant [eqns. (4)–(10)] and with a large excess of
H2O2 [eqns. (11)–(16)]. A possible mechanism is presented
in Scheme 1.
Table 2 Selected bond length (A) and angles (1) for anions 2 and 4
2 (M = Mo)
4 (M = W)
M–O1
M–O3
2.017(2)
1.948(2)
1.910(2)
2.395(2)
1.909(2)
1.953(2)
1.684(2)
44.1(1)
1.996(8)
1.879(11)
1.947(9)
2.364(10)
1.937(9)
1.857(11)
1.688(8)
45.6(4)
M–O5
M–N1
M–O2
M–O4
M–O6
O2–M–O3
O1–M–N1
O4–M–N1
O4–M–O5
O3–M–O6
O6–M–N1
72.6(1)
78.0(1)
73.0(3)
84.3(4)
44.3(1)
101.7(1)
166.0(1)
47.8(5)
105.2(4)
165.3(4)
558
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Table 3 Catalytic oxidation of various alcohols, sulfides and amines in refluxing acetonitrile (78 1C) using 2 or 4 as catalysts and H2O2 or
H2O2 þ O2 (values in parentheses) as oxidants.a
b
% Yield
TONc
Entry
1
Substrate
Time/h
24 (19)
Product
2
4
2
4
81 (84)
85 (86)
810 (840)
850 (860)
2
3
15 (12)
14 (11)
84 (81)
83 (82)
89 (84)
86 (85)
840 (810)
830 (820)
890 (840)
860 (850)
4
5
CH3OH
13 (10)
13 (10)
HCHO (4a) +
HCOOH (4b)
46 (52) +
34 (43)
48 (53) +
33 (45)
460 (520) +
340 (430)
480 (530) +
330 (450)
48 (51) +
28 (33)
51 (51) +
29 (37)
480 (510) +
280 (330)
510 (510) +
290 (370)
6
12 (9)
56 (58) +
22 (24)
59 (59) +
23 (27)
560 (580) +
220 (240)
590 (590) +
230 (270)
7
8
11 (9)
84 (91)
87 (94)
840 (910)
870 (940)
14 (11)
60 (62) +
22 (24)
62 (61) +
23 (25)
600 (620) +
220 (240)
620 (610) +
230 (250)
9
17 (13)
65 (64) +
18 (20)
68 (65) +
17 (24)
650 (640) +
180 (200)
680 (650) +
170 (240)
10
18 (15)
70 (76) +
9 (11)
72 (78) +
9 (12)
700 (760) +
90 (110)
720 (780) +
90 (120)
11
12
20 (16)
22 (22)
81 (83)
85 (86)
810 (830)
850 (860)
76 (79) +
1 (1)
79 (82) +
1 (1)
760 (790) +
10 (10)
790 (820) +
10 (10)
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559

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Table 3 (continued )
b
% Yield
TONc
Entry
13
Substrate
Time/h
12 (9)
Product
2
4
2
4
82 (80)
84 (84)
820 (800)
840 (840)
14d
1 (1)
18 (6) +
82 (94)
10 (2) +
90 (98)
180 (60) +
820 (940)
100 (20) +
900 (980)
15d
1 (1)
50 (40) +
48 (60)
46 (32) +
54 (68)
500 (400) +
480 (600)
460 (320) +
540 (680)
16d
1 (1)
54 (55) +
31 (43)
55 (52) +
33 (48)
540 (550) +
310 (430)
550 (520) +
330 (480)
17
6 (5)
57 (66) +
27 (28)
59 (70) +
28 (27)
570 (660) +
270 (280)
590 (700) +
280 (270)
18
10 (7)
62 (66) +
30 (29)
65 (70) +
32 (28)
620 (660) +
300 (290)
650 (700) +
320 (280)
19
7 (5)
82 (70) +
14 (30)
84 (86) +
15 (34)
820 (700) +
140 (300)
840 (660) +
150 (340)
a
Blank experiments were also performed, that is, eliminating the catalyst only while other parameters remaining the same. Oxidation for entries
1–3 and 8–12 was found to be negligible but for entries 4&5, 6&7, 13& 17, 14–16 and 18&19 were ca. 8%, 4%, 5%, 10% and 3%, respectively (cf.
b
c
100% = oxidation obtained using the catalyst). Based on substrate. Turnover number is defined as the ratio of the moles of product obtained
d
to the moles of catalyst used. In dichloromethane (reflux temp. 40 1C).
A possible reaction pathway with moderate excess H2O2 is
the following:
[MO(O2)2 ꢀ 2QOH] + 2eꢁ - [MO(O2)(QO)2]
+ H2O + O2ꢁ
(6)
(7)
(8)
2ꢁ
[MO(O2)(QO)2] + eꢁ - [M(O)2(QO)2] + 1/2O2
[MO(O2)(QO)2] + O22ꢁ + 2H+ - [MO(O2)2 ꢀ 2QOH]23 (4)
(5)
2ꢁ
[M(O)2(QO)2] + 1/2O2 - [MO(O2)(QO)2] + eꢁ
C6H5CH2OH - C6H5CHO + 2H+ + 2eꢁ
560
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Fig. 3 Plot of yield (based on amount of substrate) versus time for the
conversation of: (i) cyclohexane to cyclohexanone, (ii) benzyl alcohol
to benzaldehyde, (iii) cinnamyl alcohol to cinnamaldehyde and (iv)
2-octanol to 2-octanone using 4 as catalyst.
Scheme 1 Plausible mechanism of 1–4 catalytic oxidation of benzyl
alcohol to benzaldehyde showing the use of moderate and large
excesses of H2O2. Here A = a probable H2O2 associated intermediate
of [MO(O2)(QO)2] (D; 1 and 3), isolated and structurally characterized;
B is isolable (5 and 6) with the structure being drawn by inference23
;
C = [MO(O2)2(QO)]ꢁ (2 and 4), also isolated and structurally charac-
terized; E = shunt pathway for large excess of H2O2 in the reac-
tion mixture; F = shunt pathway for moderate excess of H2O2.
G ¼ [M(O)2(QO)2].
Adding eqns. (4)–(8) gives eqn. (9):
C6H5CH2OH + O2 - C6H5CHO + H2O + O2ꢁ
(9)
2ꢁ
So, the overall reaction is:
Also, it may be suggested that, when the H2O2 concentration
becomes minimal, catalytic oxidation still persists as shown in
Scheme 2, though for a very few turnovers only, and the
oxidant should then be replenished to sustain the catalytic
cycle. Intermittent addition of H2O2 is a very common proce-
dure. It minimizes side reactions, such as decomposition of
H2O2 into H2O and O2. This would not only decrease the
amount of [O22ꢁ] species in the reaction medium, but the water
formed would also have a deactivating effect on the catalysts.
All the above-mentioned experimental observations, includ-
ing catalytic and stoichiometric oxidations, are summarized in
Scheme 3. We reported23 that in the case of oxidation of methyl
benzenes with 1 as catalyst the reaction proceeded via a radical
mechanism, but in the present alcohol-to-aldehyde conversion,
the reaction proceeds via a nonradical pathway since the
reaction rate is immune to AIBN and benzoquinone. The
double additive H2O2 + O2 makes the catalyst more potent.
The exact reason is at present obscure. The oxidation products
listed in Table 3 were identified and quantified by gas chroma-
tography using di-n-butyl ether as internal standard. Notably,
in each case, we were able to separate and characterize the
reaction products as well as the catalyst from the reaction
mixture.
C6H5CH2OH + H2O2 - C6H5CHO + 2H2O
(10)
A possible reaction pathway with large excess H2O2 is the
following:
[MO(O2)(QO)2] + O22ꢁ + 2H+ - [MO(O2)2 ꢀ 2QOH]22 (11)
[MO(O2)2 ꢀ 2QOH] - [MO(O2)2(QO)]ꢁ 39
(12)
+ QOH + H+
C6H5CH2OH - C6H5CHO + 2H+ + 2eꢁ
(13)
[MO(O2)2(QO)]ꢁ + QOH + 3H+ + 2eꢁ
[MO(O2)(QO)2] + 2H2O
-
(14)
Adding eqns. (11)–(14) gives eqn. (15):
C6H5CH2OH + O22ꢁ + 2H+ - C6H5CHO + 2H2O (15)
So, the reaction equation is:
C6H5CH2OH + H2O2 - C6H5CHO + 2H2O
(16)
Fig. 4 Plot of yield (based on amount of substrate) versus time for the
conversation of: (1a) ethanol to acetaldehyde, (1b) ethanol to acetic
acid, (2) phenol to quinone, (3a) aniline to nitrosobenzene and (3b)
aniline to nitrobenzene using 4 as catalyst.
Scheme 2 Catalytic oxidation of benzyl alcohol to benzaldehyde at
low H2O2 concentrations: 1 and 3 act as active catalysts.
N e w J . C h e m . , 2 0 0 5 , 2 9 , 5 5 4 – 5 6 3
561

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Scheme 3 Schematic representation of experimental observations including stoichiometric and catalytic oxidation of benzyl alcohol to
benzaldehyde using catalysts 1–4 (here 2 and 4 are representative cases) in the presence of H2O2.
Chemical Biology, Calcutta, for use of the GC facilities.
KMAM is thankful to EPSRC for support of the X-ray
facilities at Cardiff University.
Concluding remarks
A solution obtained by dissolving MO3 (M = Mo, W) in H2O2
when treated with 8-quinolinol, gives [MO(O2)2 ꢀ 2QOH] (B) is
formed and this on crystallization affords [MO(O2)(QO)2] (D),
the catalyst precursor for oxidizing aromatic and aliphatic
alcohols including sulfides and amines (for details, see text).
Interestingly, the monoperoxo species can be obtained first by
References
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27 A. C. T. North, D. C. Phillips and F. S. Mathews, Acta Crystal-
logr., Sect. A, 1968, 24, 351.
38 (a) P. Sykes, A Guide Book to Mechanisms in Organic Chemistry,
Orient Longman Ltd., New Delhi, India, 1998; (b) W. Adam, in
Peroxide Chemistry-Mechanistic and Preparative Aspects of Oxy-
gen Transfer, ed. W. Adam, Wiley–VCH, Weinheim, Germany,
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39 This product is formed only when a large excess of H2O2 is present
in the reaction medium. This indicates the existence of another
pair of equations, between eqn. (11) and eqn. (12), involving the
formation of an activated complex, but at present the exact
interpretation remains obscure..
28 N. P. C. Walker and D. Stuart, Acta Crystallogr., Sect. A, 1983,
39, 158.
N e w J . C h e m . , 2 0 0 5 , 2 9 , 5 5 4 – 5 6 3
563