Cu(II)-Mediated decomposition of phosphorothionate P=S pesticides. Billion-fold acceleration of the methanolysis of fenitrothion promoted by a simple Cu(II)-ligand system

Article abstract of DOI:10.1039/b404740k

The kinetics of methanolysis of the title compound (3) were studied in the presence of Cu²⁺, introduced as Cu(OTf)₂, in the presence of 0.5-1.0 eq. of methoxide and in the presence of 1.0 eq. of a ligand such as bipyridyl (5), phenanthroline (6) or 1,5,9-triazacyclododecane (4). In all cases the active species involve Cu²⁺(^-OCH₃). In the case of added strongbinding ligands 5 or 6, a plot of the observed rate constant for methanolysis of 3 vs. [Cu²⁺]_total gives a curved line modelled by a process having a [Cu²⁺]^1/2 dependence consistent with an active monomeric species in equilibrium with an inactive dirtier i.e. {LCu²⁺(^-OCH₃)} ₂→2LCu²⁺(^-OCH₃). In the case of the added strong binding ligand 4, the plot of the observed rate constant for methanolysis of 3 vs. [Cu²⁺]_total gives a straight line consistent with the catalytically active species being 4Cu ²⁺(OCH₃) which shows no propensity to form inactive dimers. Turnover experiments where the [3] > [Cu²⁺] _total indicate that the systems are truly catalytic. In the optimum case a catalytic system comprising 1 mM of the complex 4Cu²⁺( ^-OCH₃) catalyzes the methanolysis of 3 with a t _1/2 of ～58 s accounting for a 1.7 × 10⁹-fold acceleration relative to the background reaction at near neutral _s^spH (8.75).

Full text of DOI:10.1039/b404740k

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A R T I C L E
Cu(II)-Mediated decomposition of phosphorothionate PS
pesticides. Billion-fold acceleration of the methanolysis of
fenitrothion promoted by a simple Cu(II)–ligand system
Alexei A. Neverov and R. Stan Brown*
Department of Chemistry, Queen’s University, Kingston, Ontario, Canada K7L 3C1.
E-mail: rsbrown@chem.queensu.ca
Received 30th March 2004, Accepted 9th June 2004
First published as an Advance Article on the web 14th July 2004
The kinetics of methanolysis of the title compound (3) were studied in the presence of Cu2+, introduced as Cu(OTf)2, in
the presence of 0.5–1.0 eq. of methoxide and in the presence of 1.0 eq. of a ligand such as bipyridyl (5), phenanthroline
(6) or 1,5,9-triazacyclododecane (4). In all cases the active species involve Cu2+(−OCH3). In the case of added strong-
binding ligands 5 or 6, a plot of the observed rate constant for methanolysis of 3 vs. [Cu2+]total gives a curved line modelled
by a process having a [Cu2+]1/2 dependence consistent with an active monomeric species in equilibrium with an inactive
dimer i.e. {LCu2+(−OCH3)}2  2LCu2+(−OCH3). In the case of the added strong binding ligand 4, the plot of the observed
rate constant for methanolysis of 3 vs. [Cu2+]total gives a straight line consistent with the catalytically active species being
4Cu2+(OCH3) which shows no propensity to form inactive dimers. Turnover experiments where the [3] > [Cu2+]total indicate
that the systems are truly catalytic. In the optimum case a catalytic system comprising 1 mM of the complex 4Cu2+(−OCH3)
catalyzes the methanolysis of 3 with a t1/2 of ~58 s accounting for a 1.7 × 109-fold acceleration relative to the background
reaction at near neutral sspH (8.75).
conditions (in methanol) is accelerated by 109-fold at ambient
Introduction
temperature in the presence of 2 mmol dm−3 La(OTf)3 along with
Phosphorothionate esters of general structure 1 (X and Y are alk-
1 eq. of NaOCH3.7a However La3+ shows no propensity to acceler-
oxy, Z is aryl with HOZ pKa between 6 and 8) are widely used
ate the methanolysis of the PS triester fenitrothion (3), probably
agricultural pesticides because they exhibit high insecticidal and
due to the “hardness” of this metal ion which resists binding to the
acaricidal properties but much lower mammalian toxicity than their
“soft” sulfur of that substrate. More recent studies have shown that
PO counterparts.1,2 Controlled decomposition of these materials is
a “softer” system, Zn2+(−OCH3), when complexed to some N-con-
of both fundamental and practical interest and generally is effected
taining ligands, reacts with both 2 and 3, although less effectively
by oxidation or hydrolysis with catalytic methods being especially
with the latter than with 2 by a factor of 17.7b Herein we report
valuable. For practical applicability a viable catalytic decomposi-
that the softer metal ion system comprising Cu2+(−OCH3) at 25 °C
tion method must be inexpensive, have high turnovers, occur at
either alone or in the presence of equimolar 1,5,9-triazacyclodo-
relatively neutral pH and ambient temperature, convert the toxic
decane (4), bipyridyl (5) or phenanthroline (6) shows both greater
starting material into far less toxic products and proceed rapidly,
catalytic efficacy and specificity toward the PS derivatives. In the
e.g. t1/2 < 1 min. Transition metal ions and lanthanides and certain
optimum case a catalytic system comprising 1 mM of the complex
mono- and dinuclear complexes thereof are known to promote the
4Cu2+(−OCH3) catalyzes the methanolysis of 3 with a t1/2 of ~58 s ac-
hydrolysis of neutral phosphate and/or phosphonate esters.3,4 How-
counting for a 1.7 × 109-fold acceleration relative to the background
ever, the available literature on the hydrolysis of the PS esters
reaction at near neutral sspH (8.75).8,9
is quite sparse with only the softer metal ions such as Cu2+, Pb2+,
Hg2+ and some cyclometallated Pt and Pd aryl ketoximes showing
significant catalysis.5 The lack of reported hydrolytic examples is
possibly due to the reduced activity of the PS esters relative to
their PO counterparts,5a,d,e their poor aqueous solubility and the
fact that the anionic hydrolytic products can bind to the metal ions
thereby curtailing further catalysis.
Recently we presented an alternative methodology for the de-
composition of phosphorus triesters such as paraoxon (2),6 namely
metal-ion catalyzed alcoholysis.7 Several reports have listed alco-
hols as co-solvents for the metal-catalyzed hydrolysis of phosphate
triesters to enhance the solubility of the reactants without altering
very much the overall kinetics, but as far as we know, none, other
than our own studies,7 has reported the pure alcohol as a reactive
medium. The catalytic alcoholysis methodology offers decided ad-
vantages over hydrolysis mainly from the perspective of substrate
solubility, enhanced Mx+:substrate pre-equilibrium complexation
leading to increased reaction rates, and the fact that the products
are also neutral triesters which are non-inhibitory. We have shown
that the methanolysis of paraoxon (2) at essentially neutral pH
Results
(a) Kinetics of methanolysis of 2 and 3
Shown in Figs. 1 and 2 are plots of kobs for the methanolysis of 3
(3 and 2 in the case of Fig. 2) vs. total amount of Cu2+ ion ([Cu2+]t)
which fall into two categories depending on the nature of the ligand
s
s
employed. In order to self-buffer the pH of the solutions, in all
cases the [−OCH3]/ [Cu2+]t ratio was kept at 0.5 corresponding to
the half neutralization point for titration of the first metal-bound
HOCH3. In the absence of any ligand or in the presence of equimo-
lar 5 or 6, the plots shown in Fig. 1 are bowed, not due to any pre-
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 2 0 0 4
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 2 2 4 5 – 2 2 4 8
2 2 4 5

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Table 1 Kinetic constants for the methanolysis of 2 and 3 catalyzed by Cu2+ in the absence and presence of ligands 4–6, T = 25 °C
Catalyst
sspH at 0.5 eq. of base
Kdisa/mmol dm−3
km(2)a/dm3 mol−1 s−1
km(3)a/dm3mol−1 s−1
Relative selectivityb
−OCH3
N.A.
1.1 × 10−2
0.22 ± 0.02
<0.2
(7.2 ± 0.2) × 10−4
0.79 ± 0.03
4.48 ± 0.12
2.44 ± 0.06
12.2 ± 0.4
1
55
Cu2+(−OCH3)c
5Cu2+(−OCH3)d
6Cu2+(−OCH3)e
4Cu2+(−OCH3)f
4Zn2+(−OCH3)g
6.86 ± 0.2
7.8 ± 0.2
7.45 ± 0.2
8.75 ± 0.1
9.3
<0.005
<0.005
<0.005
N.A.
N.A.
N.A.
342
186
67
0.86
~0
<0.2
2.76 ± 0.17
0.85 ± 0.01
47.2 ± 2.3
(4.8 ± 0.2) × 10−2
La3+2(−OCH3)2
h
i
a Dimer dissociation constant (Kdis) and conditional second order rate constant (km) for reaction with monomer defined as in text. N.A. means non-applicable
since there is no observable dimerization under the specific conditions. AKdis of <0.005 indicates very strong dimerization and is quoted as an upper limit based
on an iterative fitting procedure which provided the lowest standard deviations. b defined as (km(3)/kOCH (3))/(km(2)/kOCH (2)). c Based on fits of kobs vs. [Cu2+]t
3
data to eqn. (1) at a [methoxide]/[Cu2+]t ratio of 0.5. d Based on fits of kobs vs. [5Cu2+]t data to eqn. (1) at a3[methoxide]/[Cu2+]t ratio of 0.5. e Based on fits of kobs
vs. [6Cu2+]t data to eqn. (1) at a [methoxide]/[Cu2+]t ratio of 0.5. f Based on linear fits of kobs vs. [4Cu2+(−OCH3)]t data at a [methoxide]/[Cu2+]t ratio of 0.5. g From
ref. 7b. h From ref. 7a. i No catalysis observed.
equilibrium substrate–Cu2+ saturation binding process but rather
to a monomer–dimer equilibrium behaviour of the free or ligand
bound Cu2+(−OCH3) species. Such a monomer/dimer behaviour
requires a square-root dependence3i,n,u,7b,10 for the kobs vs. [Cu2+]t data
which can be fit via a standard NLLSQ treatment to eqn. (1) derived
on the assumptions that: (a) when present, all the ligand is bound to
Cu2+;4f and (b) an inactive dimer is in rapid equilibrium (dissociation
constant Kdis) with an active (rate constant km) monomeric species
LCu2+(−OCH3) as shown in eqn. (2). The goodness of fit of the rate
constant data to this model is shown as the computed lines through
the Fig. 1 data and the best fit constants are given in Table 1. Also
in Table 1 are the measured sspH values over the entire [Cu2+] range
under the self-buffering conditions described above which deviate
by an acceptable 0.2 or less units. In the case of 5Cu2+(−OCH3) and
6Cu2+(−OCH3) mediated methanolysis of paraoxon (2), the cata-
lyzed reactions were sufficiently slow that we have placed upper
limits on the rate and equilibrium constants.
Fig. 2 Plots of the kobs vs. total [Cu(OTf)2] for the methanolysis of para-
oxon (2, ●) and fenitrothion (3, ■) catalyzed by 4Cu2+(−OCH3) conducted at
T = 25 °C, [−OCH3]/[Cu2+]t = 0.5, [Cu2+]t = [4].
8[Cu2+
]
t
(b) NMR turnover experiment
kobs ={kmKdis ( 1+
−1)/ 4}
(1)
(2)
Kdis
To test for true catalysis, NMR experiments were performed on a
CD3OD–CH3OH (9:1) solution containing 3.6 mg (23 mmol dm−3)
of 3 and 1.78 mmol dm−3 in each of Cu(OTf)2, Bu4NOCH3 and li-
gand 4, so that the final concentration of 3 was 12.9 times in excess
of Cu2+. Almost immediately the solution became a bright yellow
colour which intensified further with time. The H and 31P NMR
1
spectra were recorded after 100 min and showed no traces (<1%) of
starting material. The only signals present belonged to the product
O,O,O-trimethyl phosphorothionate (or O,O-dimethyl O-trideu-
teriomethyl phosphorothionate): 31P NMR signal at  73.28 ppm
(lit.11 73.91 ppm). By way of reference, the 31P NMR signal for
1
unreacted 3 occurs at  66.11 ppm.11 After 100 min the H NMR
spectrum showed no (<1%) characteristic signals for fenitrothion
(aromatic protons  8.04, 7.14 and CH3O–  3.89 ppm) but did
show the characteristic signal for the product CH3O–;  3.735 ppm,
JH–P = 13.39 Hz (lit.12  3.72 ppm, JH–P = 13.45 Hz). The signals for
the aromatic protons were shifted upfield from the starting material
and were reminiscent of those for 3-methyl-4-nitrophenol but with
significant broadening, probably due to transient coordination with
Cu2+ ions.
(c) Electrospray mass spectrometric (ES-MS) study of
Cu(OTf)2 solutions with or without complexing ligands and
base
Fig. 1 Plot of the kobs vs. total [Cu(OTf)2]total for the methanolysis of
fenitrothion (3) catalyzed by various species conducted at T = 25 °C,
[−OCH3]/[Cu2+]t = 0.5; when ligand is used, [Cu2+]t = [L]: Cu2+(−OCH3) (●),
5Cu2+(−OCH3) (♦), 6Cu2+(−OCH3) (■). Lines through the data are computed
on the basis of fits to eqn. (1).
A solution containing 2 mmol dm−3 of Cu(OTf)2 in methanol
without added base was analyzed by ES-MS methods at cone volt-
ages varying from 12 to 61 V. Peaks corresponding to monomeric
(Cu(OTf))+ and dimeric forms with different numbers of solvating
molecules of methanol (Cu2(OTf)3(CH3OH)n)+ were found to be
present with n = 1–4 (at 29 V cone voltage) and n = 3–5 (at 12 V
cone voltage).
In the case of ligand 4 the behaviour is markedly different, and
the kobs vs. [Cu2+]t plots for methanolysis of both 2 and 3 shown
in Fig. 2 are strictly linear, indicative of complete formation of a
kinetically active monomeric form 4Cu2+(−OCH3). The km(2) and
km(3) second-order rate constants are evaluated as the gradients of
the linear plots and are given in Table 1.
The analysis of the ES-MS of a 2 mmol dm−3 solution of Cu(OTf)2
in the presence of 2 mmol dm−3 Bu4NOCH3 showed that the major
2 2 4 6
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 2 2 4 5 – 2 2 4 8

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species are dimeric forms (Cu2(OCH3)2(OTf)(CH3OH)n)+ with n = 2
at 14 V and n = 1 at 29 V. No monomeric species (Cu(OTf))+ or
(Cu(OCH3))+ were observed.
during the catalytic cycle. When the ligand is the triazacrown 4, we
do not see evidence of any inactive dimeric forms and the kinet-
ics indicate that the great bulk of the material is an already active
monomer (4Cu2+(−OCH3)) which may be four-coordinate in the rest-
ing state and at least five-coordinate in the transition state formed
after the transient coordination of the substrate. In Scheme 1 the
transition state involves both a Lewis acid role for the Cu2+ and si-
multaneous delivery of a coordinated methoxide. Breakdown of the
pentacoordinated phosphorus intermediate may or may not involve
assistance through coordination of the leaving group to the metal
ion: departure of a good leaving group such as p-nitrophenoxy prob-
ably does not require such assistance.15
The ES-MS of a sample containing 1 mmol dm−3 in each of
Cu(OTf)2 and bipyridyl 5 showed the presence of the dimeric spe-
cies (Cu2(5)2(OTf)3)+ as the major component and the monomeric
forms (Cu(5)(OTf))+ and (Cu(5)(OCH3))+ as minor components at
a cone voltage of 59 V: also present at this high cone voltage was
(Cu(5))+ indicating some reduction of the metal ion under these
conditions. The ES-MS of second solution formulated as above
but with 1 mmol dm−3 of added Bu4NOCH3 was also determined
at 59 V and showed the presence of (Cu2(5)2(OCH3)2(OTf))+ and
(Cu2(5)(OCH3)2(OTf))+and (Cu(5)(OCH3))+ It is of note that in the
presence of ligand one does not see the presence of methanol sol-
vated forms which are observed in the absence of ligand.
Discussion
There are several points of note revealed by the present study that
pertain to the rapid rate of catalyzed methanolysis of the PO and
PS substrates and to the stoichiometry of the catalytically active
forms. First, as is the case for uncatalyzed attack of HO− on para-
oxon (2) vs. parathion (the PS analogue of 2),13 uncatalyzed attack
of methoxide on 2 is faster (by some 15 times) than on 3. However
all the Cu2+(−OCH3) catalysts are more effective for 3 than 2. We
quantify this by the relative selectivity (RS) parameter given in
Table 1 which compares the relative reactivity of the Cu2+(−OCH3)
reaction and free methoxide attack on the PS and PO substrates.
The RS parameters clearly correlate with the hard/soft properties
of the metal ion. The “hard” ion La3+ exhibits exclusive selectivity
for the PO substrate (RS ~ 0), while the softer Zn2+ ion shows
almost equal affinity for PO and PS substrates (RS ~ 1). Of the
three ions Cu2+ is softest and exhibits very high selectivities for
the PS substrates with RS values of ~55–340, the highest values
being exhibited in the case of the aromatic ligands. Interestingly, the
best combination of selectivity and overall high catalytic activity
is achieved with 4Cu2+(−OCH3) because its extant coordination ap-
pears to prevent the undesirable dimerization exhibited by the other
Cu2+-containing catalysts.
Second, the fact that all the Cu2+-catalyzed reactions proceed with
computed second-order rate constants larger than those for the at-
tack of methoxide on 2 or 3 indicates that there is a dual role for the
metal ion. As in other Mx+-promoted hydrolytic3,4 and methanolytic7
reactions the metal ion is reasonably proposed to simultaneously
deliver a Mx+-coordinated OH− or CH3O− and act as a Lewis acid to
polarize a PS or PO unit which provides both rate and selectiv-
ity enhancement. As far as we know the 17000-fold enhancement
of attack of 4Cu2+(−OCH3) vs. free −OCH3 on 3, even though unco-
ordinated methoxide is ~108-fold more basic,14 represents the larg-
est acceleration reported for any metal-ion catalyzed phosphoryl
transfer reactions to solvent, and is even larger than the 4300-fold
enhancement that we reported earlier for attack of La3+2(−OCH3)2 vs.
free −OCH3 on paraoxon.7b
Scheme 1 L = 4, 5 or 6.
Conclusion
With due consideration for matching the hard/soft characteristics
of the substrate and the metal ion, dramatic rate and selectivity can
be achieved in the methanolysis of PO vs. PS phosphates. The
Cu2+(−OCH3) systems herein provide spectacular catalysis of the
decomposition of fenitrothion. Thus, a 1 mmol dm−3 solution of
complex 4Cu2+(−OCH3), simply formulated in situ by sequential
addition of 1 mmol dm−3 each of Cu(OTf)2, ligand 4 and methoxide
(added either as NaOCH3 or Bu4NOCH3) catalyzes the methanoly-
sis of 3 with a t1/2 of ~58 s accounting for a 1.7 × 109-fold accelera-
tion of the reaction at near neutral sspH (8.75). In systems where the
hydrolysis of neutral p-nitrophenyl phosphates was reported to be
catalyzed by metal ions the rate enhancements are considerably less,
in some cases being only 10-fold and in the best cases no more than
104–106-fold, and many of these systems are not entirely catalytic
due to lack of turnover.3,4 There are, to our knowledge, only two
studies where the hydrolysis of phosphorothionates is reported to
have billion-fold catalysis relative to the background, namely when
promoted by cycloplatinated or cyclopalladated aryl ketoximes.5d,e
Experimental
(a) Materials
Methanol (99.8% anhydrous), sodium methoxide (0.5 M solution
in methanol), Bu4NOH (1 M solution in methanol: at the diluted
mmol dm−3 concentrations used in our experiments, we assume
that the water content can be ignored and that the active base is
Bu4NOCH3), copper perchlorate (Cu(ClO4)2), paraoxon (2), bipyri-
dyl (5), phenanthroline (6) and 1,5,9-triazacyclododecane (4) were
purchased from Aldrich and used without any further purification.
Copper triflate (Cu(OTf)2) was purchased from Strem Chemicals
Inc. HClO4 (70% aqueous solution) was purchased from BDH.
Fenitrothion (3) was a gift from Prof. Erwin Buncel of this depart-
ment. CAUTION: paraoxon and fenitrothion are acetylcholines-
terase inhibitors, with oral LD50 values of 1.8 and 250 mg kg−1,
respectively, in rats.16
It is also of note that we have, through turnover experiments,
demonstrated that this is a truly catalytic system which, at concen-
trations of 1 mmol dm−3, can provide 1.7 × 109-fold acceleration of
the methanolysis of 3 at neutral sspH and ambient temperature. This
has been demonstrated unambiguously by the NMR experiments
on the 4Cu2+(−OCH3) system which is capable of at least 12 turn-
overs under the conditions employed. These findings supplement
our previous studies7 that effective true catalysts for alcoholysis of
phosphate triesters can be generated in situ from metal triflate salts,
an appropriate ligand, and the addition of up to one eq. of methoxide
per Mx+.
Presented in Scheme 1 is a proposed mechanism for the catalytic
methanolysis reaction of 3 promoted by LCu2+(−OCH3). Depending
on the ligand chosen, for example, bipyridyl 5 or phenanthroline
6, the predominant form in solution is an unreactive dimer which
must dissociate to an active monomer LCu2+(−OCH3) which is prob-
ably square planar with a weakly associated HOCH3 which can be
replaced by the XP of the substrate to form a transient complex
(b) Methods
1H NMR spectra were determined at 400 MHz and referenced to
the CD2H peak of d4-methanol appearing at  3.31 ppm. 31P NMR
spectra were referenced to an external standard of 70% phosphoric
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 2 2 4 5 – 2 2 4 8
2 2 4 7

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acid in water, and upfield chemical shifts are negative. Mass-spectra
of 1–2 mmol dm−3 solutions of Cu(OTf)2 in methanol in the pres-
ence of ligands and base were determined using a VG Quattro mass
spectrometer equipped with an electrospray source operating at
+
cone voltages between 12 and 60 V. The CH3OH2 concentration
was determined using an auto-titrator equipped with a Metrohm
6.0255.100 combination (glass/calomel) electrode calibrated with
standardized aqueous buffers (pH = 4.00 and 10.00) as described in
our recent papers.7,17 Values of sspH were calculated by subtracting
a correction constant of −2.24 from the experimental meter reading
as reported by Bosch et al.18
(c) Kinetics
The kinetics of methanolysis were monitored at 25 °C in anhydrous
methanol by observing the rate of appearance of p-nitrophenol
or 3-methyl-4-nitrophenol between 312 and 335 nm at [2] or
[3] = (4–12) × 10−5 mol dm−3 under pseudo-first-order conditions
of excess Cu(OTf)2 ((0.2–5.0) × 10−3 mol dm−3). All reactions
were followed to at least three half lives and found to exhibit good
pseudo-first order rate behavior and the first-order rate constants
(kobs) were evaluated by least squares fitting the Abs. vs. time traces
to a standard exponential model. The kinetics were all determined
s
s
under self-buffered conditions where the pH was controlled by
6 CAUTION: paraoxon and fenitrothion are potent acetylcholinesterase
inhibitors and should be used with appropriate care avoiding contact and
inhalation.
7 (a) J. A. W. Tsang, A. A. Neverov and R. S. Brown, J. Am. Chem. Soc.,
2003, 125, 7602; (b) W. Desloges, A. A. Neverov and R. S. Brown,
Inorg. Chem., in press.
a constant Cu2+/Cu2+(−OCH3) ratio and in the cases with ligands
4–6, these were added in amounts equivalent to the [Cu2+]total
.
s
Under these conditions, the observed pH values correspond to
s
s
s
the apparent pHa value for ionization of the LCu2+(HOCH3) 
LCu2+(−OCH3) + +H2OCH3 system. A turnover experiment was
conducted using 0.4 mM Cu(OTf)2 along with equimolar 4 and 0.5
eq. of NBu4OCH3. The methanolysis of 2 mM 3 was monitored
by UV/vis kinetics and showed 10 turnovers relative to the active
catalyst (0.2 mM 4Cu2+(−OCH3)) within 100 min.
8 For the designation of pH in non-aqueous solvents we use the forms
described by Bosch and co-workers9,18 based on the recommendations
of the IUPAC, Compendium of Analytical Nomenclature. Definitive
Rules 1997, Blackwell, Oxford, UK, 3rd edn., 1998. If one calibrates
the measuring electrode with aqueous buffers and then measures the pH
of an aqueous buffer solution, the term wwpH is used; if the electrode is
calibrated in water and the ‘pH’ of the neat buffered methanol solution
then measured, the term ws pH is used; and if the latter reading is made,
and the correction factor of 2.24 (in the case of methanol) is added, then
the term sspH is used.
Acknowledgements
The authors thank the Natural Sciences and Engineering Research
Council of Canada and Queen’s University for the support of this
work.
9 Given that the autoprotolysis constant of methanol is 10−16.77, neutral sspH
in methanol is 8.4; E. Bosch, F. Rived, M. Rosés and J. Sales, J. Chem.
Soc., Perkin Trans. 2, 1999, 1953.
10 (a) E. Arenare, P. Paoletti, A. Dei and A. Vacca, J. Chem. Soc., Dalton
Trans., 1972, 736; (b) J. G. J. Weijnen, A. Koudijs, G. A. Schellekens
and J. F. J. Engbersen, J. Chem. Soc., Perkin Trans. 2, 1992, 829;
(c) J. G. J. Weijnen, A. Koudijs and J. F. J. Engbersen, J. Org. Chem.,
1992, 57, 7258.
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2 2 4 8
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 2 2 4 5 – 2 2 4 8