Use of MoO2Cl2(DMF)2 as a precursor for molybdate promoted hydrolysis of phosphoester bonds

Article abstract of DOI:10.1039/c2dt32734a

Phosphoester bond cleavage of para-nitrophenylphosphate (pNPP), a commonly used model substrate, is accelerated by using the complex MoO₂Cl ₂(DMF)₂ (1) (DMF = dimethylformamide) as a hydrolysis promoting agent, even when c

Full text of DOI:10.1039/c2dt32734a

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Use of MoO2Cl2(DMF)2 as a precursor for molybdate
promoted hydrolysis of phosphoester bonds
Cite this: Dalton Trans., 2013, 42, 3901
Cátia M. Tomé,a,b M. Conceição Oliveira,b Martyn Pillinger,a Isabel S. Gonçalvesa and
Marta Abrantes*b
Phosphoester bond cleavage of para-nitrophenylphosphate (pNPP), a commonly used model substrate, is
accelerated by using the complex MoO2Cl2(DMF)2 (1) (DMF = dimethylformamide) as a hydrolysis pro-
moting agent, even when catalytic amounts of 1 (10 mol% relative to pNPP) are used. The reactions were
performed under mild conditions (37–75 °C) and followed by 1H NMR spectroscopy. For assays performed
with high amounts of 1 (1000 mol% relative to pNPP), a white solid (2) precipitates during the initial
stages of the reaction, which subsequently dissolves, leading eventually to the precipitation of a less
soluble yellow solid (3). Taken together, the characterization data for 2 (FT-IR spectroscopy, elemental
analysis, 1H and 13C NMR, and electrospray ionization mass spectrometry) indicate that it is a polymeric
material with the formula Mo2O6(DMF)n and a structure comprising infinite isopolyoxomolybdate chains
built up from edge-shared {MoO6} octahedra. Compound 3 was identified as the Keggin-type phospho-
molybdate [(CH3)2NH2]3PMo12O40. The formation of 3 is explained by the reaction of inorganic phos-
phate ions with isopolymolybdate species derived from 2, with dimethylammonium ions arising from the
degradation of DMF. Both 2 and 3 are active for phosphoester bond hydrolysis with conversion profiles
comparable to the ones obtained with the precursor 1.
Received 15th November 2012,
Accepted 14th December 2012
DOI: 10.1039/c2dt32734a
www.rsc.org/dalton
accumulation in the environment is a recognized ecological
threat with harmful effects on human beings or other mam-
Introduction
Phosphoesters play major structural and mechanistic roles in malian species due to possible long-term exposure to sub-
nature. They are the basis of energy formation and consump- lethal doses.4,5
tion, the center of phosphorylation processes and crucially
Interest in developing systems that promote phosphoester
form the backbone of both nucleic acids (RNA and DNA).1 bond cleavage is, therefore, related to three different important
Besides being ubiquitous in many important biological pro- topics: enzyme phosphatase mimicking, inactivation of organo-
cesses, phosphoester bond cleavage reactions are the key trans- phosphorous warfare agents and inactivation of organophos-
formation in the degradation of toxic organophosphorous phorous pesticides.3,6–8
compounds, such as certain classes of chemical warfare
agents or pesticides.2
Over time a growing interest in the metal promoted hydro-
lysis of phosphate esters has arisen. In recent years, a range of
Despite intense research during the second half of the last positively charged metal complexes containing Cu2+, Ln3+,
century, the decontamination and demilitarization of organo- Co3+, Zn2+, Mn2+, Fe3+ and Zr4+ has proven to exhibit high
phosphorous warfare agents is still, unfortunately, an impor- activity toward phosphoester cleavage.3,6
tant problem to address.3,4 Furthermore, although
To the best of our knowledge, the use of molybdenum as a
organophosphorous pesticides are of extreme importance to hydrolysis promoting agent has been limited to molybdocene
the agricultural industry (having guaranteed that millions of derivatives (Cp′2MoCl2)2,9–11 and to molybdates (MoO42−),12–19
people have access to food), their extensive use can also lead to even though molybdenum is known to be an interesting metal
poisoning due to the occupational or incidental exposure to for a variety of biological and catalytic applications.20,21 For
large numbers of people worldwide.4,5 Moreover, their molybdocene derivatives, the reactive species were proposed to
be Cp′2Mo(OH)(H2O)+, which are formed by reaction of
Cp′2MoCl2 with the reaction solvent (water). Phosphoester
aDepartment of Chemistry, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal
hydrolysis was described as promoted and not catalyzed by
bCentro de Química Estrutural, Complexo Interdisciplinar, Instituto Superior
molybdocenes.2,11
Técnico, Universidade Técnica de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa,
The effect of molybdates on phosphoester hydrolysis was
originally studied in the context of analytical colorimetric
Portugal. E-mail: marta.abrantes@ist.utl.pt; Fax: +351 21 846 4455;
Tel: +351 21 841 9264
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methods for organic phosphate estimation.17–19 However, (UltraShieldTM Magnet) spectrometers at ambient tempera-
1
recently, a growing interest in the biological activity of molyb- ture. For H and 13C, deuterated solvents were used as a refer-
dates and their interaction with phosphate-containing biomo- ence (except in the case of the 13C NMR performed in D2O in
lecules has arisen.12–16,22 For molybdates, reactive species which MeOH was used as an external reference). For 31P NMR,
responsible for phosphoester hydrolysis were proposed to be concentrated H3PO4 (85%) was used as a reference.
polyoxomolybdates that arise from the solubility of the molyb-
ESI mass spectra were obtained on a 500-MS quadrupole
date in water under the different pH ranges studied. Interest- ion trap mass spectrometer (Varian Inc., Palo Alto, CA, USA).
ingly, these last results have altered the paradigm of metal Samples were introduced into the ESI ion source using a
complex promoted hydrolysis which assumed that only posi- syringe pump set to a flow rate of 20 μL min−1. The ion spray
tively charged unsaturated complexes could act as hydrolysis voltage was set at 5 kV; capillary voltage: 60–80 V and RF
promoters. Results obtained with molybdates showed that loading 80%. Nitrogen was used as the nebulizing and drying
negatively charged and saturated complexes could also gas, at pressures of 35 psi and 10 psi, respectively; drying gas
efficiently hydrolyze phosphoester bonds.15 Molybdates have temperature = 350 °C. The spectra were recorded in the range
also been reported to have an impact on the hydrolytic activity of 100–1000 Da. Spectra typically correspond to the average of
of phosphatase enzymes. Both the effects of competitive 20–35 scans. The sample was dissolved in Millipore deionized
inhibition23–27 and promotion27,28 have been described.
water immediately prior to the analysis. Theoretical isotope
Recently, some of us have devoted our attention to the reac- patterns were calculated using the ISOPRO 3.0 program.
tion of catalyst precursors of the type MoO2Cl2(L) with water
Molybdenum-promoted hydrolysis assays
under mild reaction conditions as a strategy to synthesize new
inorganic–organic molybdenum oxide hybrids for catalytic Sodium para-nitrophenylphosphate (pNPP, 3.0 mg, 8 × 10−3
applications.29,30 Besides possessing an added value in the mmol) and dioxane (internal standard, 0.8 mg, 9 × 10−3
context of sustainable chemistry, the study of the reactivity of mmol) were dissolved in deuterated water in a 5 mm NMR
metal complexes in water is very appealing because it paves tube bearing a magnetic stirring bar, and heated to the assay
the way to the vast field of biological applications. Our pre- temperature. The molybdenum compound (variable amounts
vious work revealed that reaction with water afforded a in the range of 10–100 mol% relative to pNPP) was added, and
1-dimensional polymeric hybrid for L = 2,2′-bipyridine29 and an the progression of the reaction monitored over time by 1H
octanuclear cluster for L = 4,4′-di-tert-butyl-2,2′-bipyridine,30 NMR. Attention: The magnetic stirring bar was removed
which showed interesting catalytic properties in olefin epoxi- immediately before the NMR measurement and then reintro-
dation29,30 and in oxidation of secondary amines to nitrones.31 duced immediately after the measurement. The relative
It is in this context that the present work aims at applying amounts of pNPP and para-nitrophenol (pNPh) in the reaction
the complex MoO2Cl2(DMF)2 (DMF = dimethylformamide) as a medium were followed by quantification of the respective
hydrolysis promoting agent of a model phosphate ester in areas in comparison to the area of the internal standard. The
aqueous media. MoO2Cl2(DMF)2 was chosen due to its pH of the solution was measured at the described times and
straightforward synthesis (under air, using aqueous media), reaction temperature. The pD value of the solution was
water solubility and possession of Cl ligands, which are prone obtained by adding 0.41 to the pH reading. Before addition of
to hydrolysis and substitution by oxygen ligands.
the catalyst the reaction medium has a pD value of 7.4.
Addition of the metal complexes studied causes significant
differences in the pD of the reaction medium which persist
throughout the reaction. Reactions were not buffered to under-
stand the real impact of each compound per se in the hydro-
lysis reaction.
Experimental section
Materials and methods
MoO2Cl2 (Sigma-Aldrich), diethyl ether (Sigma-Aldrich, puris,
Assays with 1 × 103 mol% molybdenum compound relative
p.a.), para-nitrophenylphosphate disodium salt hexahydrate to pNPP were performed in a 3 mL V-vial bearing a magnetic
(Alfa Aesar, 99%), D2O (Euroiso-top, 99.96%), 1,4-dioxane stirring bar and using water as a solvent. These assays were
(Fischer Chemical, 99.99%), Na2MoO4·2H2O (Fluka, >99%), performed without using a water suppression technique but
and phosphomolybdic acid hydrate (Sigma-Aldrich) were quantification was performed after local baseline correction.
obtained from commercial sources and used as received. The
Synthesis and characterization of 2
complex MoO2Cl2(DMF)2 (1) was synthesized according to the
published procedure.32
MoO2Cl2(DMF)2 (32.37 mg, 0.094 mmol) was fully dissolved in
Microanalyses (CHN) were provided by the Analysis Labora- H2O (1.6 mL) and the solution was stirred at 1000 rpm using
tory of the Instituto Superior Técnico, Technical University of an Eppendorf Thermomixer compact, at 37 °C. After 90 min a
Lisbon, and Mo contents were determined by ICP-OES at the bluish white solid precipitated. The solid was isolated by cen-
Central Laboratory for Analysis, University of Aveiro. FT-IR trifugation, washed with diethyl ether (3 × 1.5 mL), and finally
spectra were recorded as KBr pellets using a JASCO FT/IR 4100 vacuum dried. Yield: 9.23 mg, 54%. Anal. calcd for
spectrometer. 1H, 13C and 31P NMR spectra in solution were C3H7Mo2NO7 (360.97): C, 9.98; H, 1.95; N, 3.88. Found:
recorded on Bruker Avance II+ 300 and 400 MHz C, 9.97; H, 1.90; N, 3.61. Compound 2 is soluble in water and
3902 | Dalton Trans., 2013, 42, 3901–3907
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insoluble in the vast majority of organic solvents. FT-IR (KBr,
cm−1): ν = 2950 (w), 1663 (vs) (CvO from DMF), 1500 (w), 1427
(w), 1374 (m), 1247 (w), 1118 (w), 1059 (w), 975 (w), 960 (vs)
ν(MovO), 933 (vs) ν(MovO), 907 (vs) ν(MovO), 697 (w), 627
(w), 551 (vs, br) ν(OMo3), 505 (m). 1H NMR (300 MHz, D2O,
r.t.): δ = 7.92 (s, 1H, (CH3)2NCHO), 3.00 (s, 3H, (CH3)2NCHO),
2.85 (s, 3H, (CH3)2NCHO). 13C NMR (101 MHz, D2O, r.t.):
δ = 165.10 ((CH3)2NCHO), 37.05, 31.52 ((CH3)2NCHO). ESI-MS
(m/z (%)): [M + H+] = {MoO3(DMF)MoO3 + H}+ (m/z 361.9 (75%)).
Synthesis and characterization of 3
Sodium para-nitrophenylphosphate (10.2 mg, 0.027 mmol), 1
(91.4 mg, 0.265 mmol) and dioxane (internal standard,
3.90 mg, 0.044 mmol) were dissolved in water (1 mL) and the
mixture was stirred at 55 °C. After 30 min compound 2 precipi-
tated from the reaction medium. Progression of the reaction
caused the appearance of a yellow precipitate, which was iso-
lated by filtration when the hydrolysis reaction was complete
(as monitored by 1H NMR). The yellow solid was washed
with diethyl ether (1.5 mL) and vacuum dried. Yield (based
on MoO2Cl2(DMF)2): 40.8 mg, 95%. Anal. calcd for
C6H24Mo12N3O40P (1960.50): C, 3.68; H, 1.23; N, 2.14; Mo,
58.72. Found: C, 3.75; H <2; N, 1.96; Mo, 59.3. [(CH3)2NH2]3-
PMo12O40 (3) is insoluble in water. FT-IR (KBr, cm−1): ν = 3187
(w) ν(N–H), 1558 (w), 1463 (w), 1413 (w), 1148 (w), 1063 (s), 968
(vs), 960 (vs) ν(MovO), 865 (m), 796 (vs) ν(Mo–O–Mo), 701 (w).
1H NMR (300 MHz, (CD3)2SO, r.t.): δ = 8.15 (br), 2.55 (s).
31P NMR (162 MHz, (CD3)2SO, r.t.): δ = −2.92.
Fig. 1 Representative 1H NMR spectra (400 MHz, detail 6.6–8.5 ppm and
2.5–4.0 ppm) of pNPP hydrolysis reaction. Reaction conditions: 100 mol% of 1
relative to pNPP, 55 °C, D2O, dioxane as the internal standard.
Results and discussion
Sodium para-nitrophenylphosphate (pNPP) was used as a
model substrate for the phosphoester bond hydrolysis reaction
(Scheme 1). MoO2Cl2(DMF)2 (1) was used as a hydrolysis pro-
moting agent (10–1000 mol% relative to pNPP) and the reac-
tions were followed by 1H NMR spectroscopy.
Comparison of the reaction profiles with the blank reaction
(performed in the absence of any metal complex) at 55 °C
clearly shows that the hydrolysis reaction is accelerated by the
presence of the molybdenum complex 1 (Fig. 2). The blank
reaction progresses slowly, maintaining a modest hydrolysis of
10% even after 10 days. In the presence of the complex 1
(100 mol% relative to pNPP), hydrolysis of 10% is surpassed in
the first hour of reaction, increasing consistently until 100% is
reached at ca. 15 days.
Over time, and for the different studied systems, the 1H
NMR spectra of the reaction showed that the amount of pNPP
(doublets at δ 8.21 and 7.31) decreased with the concomitant
increase of the hydrolysis product para-nitrophenol (pNPh)
(doublets at δ 8.15 and 6.89) (Fig. 1).
Varying the reaction temperature has an important influ-
ence on the progression of the reaction (Fig. 3). An increase of
20 °C in the reaction temperature decreases the half-life of the
reaction by about one order of magnitude (estimated t1/2
=
400 h (35 °C), 60 h (55 °C), 5 h (75 °C)).
Interestingly, performing the reaction with catalytic
amounts of 1 (10 mol% relative to pNPP) also enables an accele-
rated and complete pNPP hydrolysis, although at a slower rate
(Fig. 2). To the best of our knowledge this is the first report of
pNPP hydrolysis catalyzed by a molybdenum complex. Previous
reports using molybdenum complexes as hydrolysis promoting
agents for pNPP were performed with excess of Mo: between
1.1 × 104 and 3.3 × 105 mol% relative to pNPP for Cp′2Mo(OH)-
(H2O)+,2,11 between 6.8 × 103 and 3.4 × 104 mol% relative to
pNPP for (NH4)2(MoO4),18 and 700 mol% relative to pNPP for
Scheme 1 Hydrolysis of para-nitrophenylphosphate (pNPP) to give para-nitro-
phenol (pNPh).
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△
○
) or phos-
Fig. 4 Hydrolysis profile of pNPP with 1 (+), 2 (×), 3 ( ), Na2MoO4
(
○
Fig. 2 Hydrolysis profile of pNPP either in the absence of 1 ( ) or with 10 (×),
□
phomolybdic acid ( ). Reaction conditions (pD values in parentheses): 100 mol%
1 (1.4), 2 (5.4), Na2MoO4 (7.4) and phosphomolybdic acid (1.4), 20 mol%
3 (5.4), 55 °C, D2O, dioxane as the internal standard.
■
100 (+) or 1000 ( ) mol% 1 relative to pNPP. Reaction conditions: 55 °C, D2O,
dioxane as the internal standard. Please refer to text for pD values.
calculated model and the experimental data, probably due to
the complexity of the system.
With the aim of understanding the above-mentioned tran-
sition from a monophasic system to a biphasic one, and of
unraveling the nature of the molybdenum species involved, 1H
NMR spectra of the hydrolysis reaction were examined for the
presence of peaks related to 1. Independent of the reaction
temperature and the concentration of 1, 1H NMR spectra of
the hydrolysis reaction showed peaks due to dimethylform-
amide (DMF) originating from the metal promoter (δ 7.91, 2.99,
2.84). Interestingly, the chemical shifts for these peaks corre-
spond to those expected for free DMF, possibly indicating that
upon dissolution in water, DMF ligands of 1 quantitatively
pass into the solution (Fig. 1).33 In fact, addition of a DMF
aliquot to the reaction system does not result in the appear-
ance of new peaks but only the increase in intensity of the pre-
viously described peaks, thus sustaining the hypothesis of the
passage of DMF ligands to solution.
□
▲
Fig. 3 Hydrolysis profile of pNPP at 35 ( ), 55 (+) and 75 °C ( ). Reaction con-
ditions: 100 mol% 1, D2O, dioxane as the internal standard.
Over time the DMF peaks lose part of their intensity and a
Na2MoO4.15 In the latter example the highest hydrolysis rates new set of singlets appears at 8.21 and 2.70 ppm (the intensi-
6−
were observed between pD 4 and 6 where Mo7O24 predomi- ties of which are proportional to the amount of 1 used in the
nates in solution. In our studies an assay performed using assay), which can be assigned to formic acid and the dimethyl-
Na2MoO4 (100 mol% relative to pNPP) showed that 1 presents ammonium cation, respectively. Under the reaction con-
a better hydrolysis promoting performance, especially after the ditions used, the hydrolysis of the Mo–Cl bonds in 1 gives HCl
half-life time point (Fig. 4). Assays performed with 1.0 × 103 and an acidic solution (at 55 °C, constant pD values up to
mol% 1 relative to pNPP showed no significant impact on the 300 h reaction time of 6.4 for 10 mol% 1 and 1.4 for 100 and
hydrolysis of pNPP (Fig. 2). At these high concentrations of a 1000 mol% 1; cf. pD = 7.4 for the blank reaction and for the
molybdenum complex, the reaction medium evolves from a reaction carried out with 100 mol% Na2MoO4), which likely
monophasic system to a biphasic solid–liquid system (after ca. accelerates the degradation of DMF to give formic acid and
30 min, see below).
dimethylammonium ions.26
Attempts to fit the data of the reaction profiles to simple
Close inspection of the 1H NMR spectra for all reactions
kinetic models, which would enable the comparison of the showed the appearance of a transitory peak at 7.80 ppm. In all
reactions in a more quantitative way, were made. Unfortu- cases this peak disappeared after the initial phase of the reac-
nately, none of the attempts resulted in a valid fit between the tion. Transitory appearance of this modest peak may be
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correlated with an intermediate species in which the metal
complex is bonded to pNPP, as reported by Absillis et al. while
using Na2MoO4 as a hydrolysis promoting agent.15
As mentioned above, for assays performed with high
amounts of 1 (1.0 × 103 mol% relative to pNPP), the system
becomes solid–liquid biphasic. The precipitate (obtained after
ca. 30 min of reaction) can be isolated as a white solid (2). This
solid is water soluble and thus its precipitation from the reac-
tion medium can be explained by its high concentration in solu-
tion, since the reaction medium is aqueous.
FT-IR analysis of 2 indicates the presence of terminal
MovO groups (sharp ν(MovO) bands at 960, 933, 907 cm−1
)
and μ3-bridging oxide groups (broad ν(OMo3) band at
551 cm−1). The absence of strong bands in the region of
700–850 cm−1 seems to exclude the existence of μ2-bridging
oxide groups (Mo–O–Mo).34 Indeed, the pattern of bands for
the Mo–O vibrations matches that typically exhibited by molyb-
denum trioxide hydrates of the type MoO3·nH2O (n = 0.5,
1.0),35 or molybdenum oxide/organic composite materials con-
taining inorganic chains built up from edge-shared distorted
{MoO6} octahedra.29,36 A band at 1663 cm−1 for 2 indicates the
presence of the coordinated carbonyl group of dimethylform-
amide (free DMF presents a band at 1675 cm−1). Elemental
analysis is consistent with the formula Mo2O6(DMF).
Fig. 6 Negative ion ESI mass spectrum of an aqueous solution of 2. The
enlarged portion of the spectrum shows the overlapping of two different distri-
bution envelopes which match a doubly charged species and a singly charged
species. Note that the isotopic separation in the mass spectrum of 0.5 m/z units
is indicative of a doubly charged ion.
isopolymolybdates to undergo aggregation and water conden-
sation reactions.37,38 The m/z values assigned to each peak are
those measured from the centroid of the ESI mass spectral
envelope and are in good agreement with the calculated m/z
value of the most abundant peak within each isotopic mass
distribution divided by the ion charge.
1
Further characterization of 2 was performed in solution. H
NMR and 13C NMR spectra obtained in deuterated water con-
firmed the presence of DMF but again with chemical shifts
characteristic of free DMF rather than coordinated DMF, prob-
ably indicating the passage of the ligand into solution.
Taken together, the characterization data for 2 lead us to
propose that it has a polymeric structure formulated as
Mo2O6(DMF)n comprising infinite isopolyoxomolybdate chains
structurally analogous to those found previously in a few
selected compounds (Scheme 2).29,36 Upon dissolution of 2 in
water, the DMF ligand passes into solution, and a series of
isopolyoxomolybdates are formed. A compound with the
same characterization profile of 2 can be obtained by the reac-
tion of 1 with water at different temperatures (room tempera-
ture, 37 °C and 55 °C; please see the Experimental section for
details).
An aqueous solution of 2 was analyzed by electrospray ion-
ization (ESI) mass spectrometry. The full-scan ESI(+) mass
spectrum of 2 (Fig. 5) shows a group of peaks centered at m/z
362 with a characteristic isotopic distribution for a MoVI-con-
taining species that agrees well with that calculated for
[(MoO3)2(DMF) + H+]. The isotopic envelope of peaks at m/z
220 (base peak) is attributed to a MoVI-containing species with
an isotopic pattern matching [MoO3(DMF) + H+]. The negative
ion ESI mass spectrum of 2 (Fig. 6) exhibits two series of
−
polyoxomolybdate anions, namely HMomO3m+1 (m = 1–6)
2−
and MomO3m+1
(m = 3–13), supporting the ability of
Scheme 2 Proposed scheme for the transformation of 1 under the reaction
conditions used for hydrolysis of pNPP (100–1000 mol% 1 relative to pNPP,
55 °C, D2O, dioxane as the internal standard).
Fig. 5 ESI(+) mass spectrum of a solution of 2 in water. The inset shows the cal-
culated isotopic pattern of (MoO3)2(C3H7NO)H+.
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Interestingly, 2 can also be used as a hydrolysis promoting expanding the range of substrates to include toxic organopho-
agent, presenting a conversion profile very similar to that for sphate esters (pesticides).
its precursor 1 (Fig. 4). Compound 2 is water soluble and origi-
nates monophasic systems when used in equimolar amounts
relative to pNPP (constant pD of 5.4 up to 300 h reaction time).
Acknowledgements
After the half-life time point of the hydrolysis reaction (for
reactions performed with 100–1000 mol% of 1 or 2 relative to
pNPP) a yellow precipitate (3) is formed. Unlike 2, 3 is insolu-
ble in the aqueous reaction medium. Characterization of 3 by
FT-IR indicates the presence of absorption bands due to P–O
We are grateful to the Fundação para a Ciência e a Tecnologia
(project no. PTDC/EQU-EQU/121677/2010), QREN, FEDER,
COMPETE, and the European Union for funding. The Associ-
ate Laboratory CICECO (Pest-C/CTM/LA0011/2011) is acknow-
ledged for continued support, including grant no. BI/UI89/
6110/2012 to C.M.T. Authors thank the project PEst-OE/QUI/
UI0100/2011, the Portuguese NMR Network (IST-UTL Center)
for providing access to the NMR facilities and the Portuguese
MS Network (IST Node) for the ESI measurements (REDE/
1502/REM/2005).
(1063 cm−1), MovO (968 cm−1
) and Mo–O–Mo (865,
796 cm−1) stretching vibrations, characteristic of Keggin-type
phosphomolybdates.39,40 Weak bands at 3187 cm−1 and
between 1462 and 1413 cm−1 can be attributed to dimethyl-
ammonium ions. In fact, the FT-IR spectrum of 3 is a close match
40
with that reported by Liu et al. for [(CH3)2NH2]3PMo12O40
.
The 1H NMR spectrum of 3 confirms the presence of dimethyl-
ammonium protons (as found during the hydrolytic process),
and the 31P NMR chemical shift is in agreement with data for
other phosphomolybdates (please see the Experimental
section).41 The appearance of phosphomolybdate can be easily
understood if one considers that inorganic phosphate species
are available in the reaction medium as the hydrolysis product
of pNPP, and can easily react with the isopolyoxomolybdate
species derived from 2, with dimethylammonium ions arising
from the degradation of DMF (see above, Scheme 2).
Notes and references
1 W. W. Cleland and A. C. Hengge, Chem. Rev., 2006, 106,
3252.
2 L. Y. Kuo, S. Kuhn and D. Ly, Inorg. Chem., 1995, 34, 5341.
3 K. Kim, O. G. Tsay, D. A. Atwood and D. G. Churchill,
Chem. Rev., 2011, 111, 5345.
Further tests have shown that 3 can be synthesized at
various temperatures (from 35 to 55 °C) for mol% of 1 relative
to pNPP > 100% and in the presence of dioxane (please see the
Experimental section for details).
Isolation of 3 intrigued us regarding its activity. Results
showed that 3 and its water soluble analogue phosphomolyb-
dic acid (PMA) are also active for phosphoester hydrolysis with
profiles comparable to those obtained with 1 and 2 (Fig. 4),
thus indicating that the phosphomolybdates are not inactive
end species but active participants in the hydrolysis process.
4 H. Morales-Rojas and R. A. Moss, Chem. Rev., 2002, 102,
2497.
5 J. Fanzo, R. Remans and P. Sanchez, in The Chemical
Element: Chemistry’s Contribution to Our Global Future, ed.
J. Garcia-Martinez and E. Serrano-Torregrosa, Wiley-VCH
Verlag, Weinheim, 2011.
6 F. Aguilar-Pérez, P. Gómez-Tagle, E. Collado-Fregoso and
A. K. Yatsimirsky, Inorg. Chem., 2006, 45, 9502.
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lics, 2007, 26, 5179.
Conclusions
We have shown that the dioxomolybdenum(VI) complex
MoO2Cl2(DMF)2 (1) can be used directly as a precursor to 12 E. Ishikawa and T. Yamase, J. Inorg. Biochem., 2006, 100,
hydrolytically active species for the phosphoester bond clea- 344.
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