Mechanism and selectivity control of methyl acetate and methyl formate formation from methanol alone with [Ru(SnCl3)5(PPh3)]3- as catalyst

Article abstract of DOI:10.1039/a606314d

Mechanistic studies have been made of the unique one-pot conversion of methanol into acetic acid (and/or methyl acetate due to rapid esterification) with [Ru(SnCl₃)₅(PPh₃)]^3- as catalyst. Addition of Cl^-1 ion promoted the formation of methyl formate relative to methyl acetate, but scarcely changed their total rate of formation. Further, the activation energies for their formation were virtually identical (≈76 kJ mol^-1). No ¹³C was incorporated into methyl acetate even when the reaction was performed under a ¹³CO atmosphere, indicating that the conversion of methanol into methyl acetate does not involve carbonylation. The effect of the addition of Cl^- ion on the reaction of formaldehyde was similar to that when using methanol as substrate, but quite different activation energies were obtained for the formation of methyl acetate (27.9 kJ mol^-1) and methyl formate (40.6 kJ mol^-1). The results are in accord with rate-determining dehydrogenation of methanol to formaldehyde (as a common intermediate), which is then converted competitively into acetic acid and methyl formate with different activation energies. The effect of added free Cl^- ion for both substrates suggests the presence of a pre-equilibrium dissociation of Cl^- from the co-ordinated SnCl₃^-, which is required for the formation of acetic acid but not for methyl formate. A possible reaction scheme is presented, in which reductive elimination of methyl formate is competitive with that of acetic acid.

Full text of DOI:10.1039/a606314d

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Mechanism and selectivity control of methyl acetate and methyl
3Ϫ
formate formation from methanol alone with [Ru(SnCl ) (PPh )] as
3
5
3
catalyst †
Takeshi Ohnishi, Tetsu Yamakawa and Sumio Shinoda *
Institute of Industrial Science, The University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106,
Japan
Mechanistic studies have been made of the unique one-pot conversion of methanol into acetic acid (and/or methyl
3
Ϫ
Ϫ
acetate due to rapid esterification) with [Ru(SnCl ) (PPh )] as catalyst. Addition of Cl ion promoted the
3
5
3
formation of methyl formate relative to methyl acetate, but scarcely changed their total rate of formation. Further,
Ϫ1
13
the activation energies for their formation were virtually identical (≈76 kJ mol ). No C was incorporated into
1
3
methyl acetate even when the reaction was performed under a CO atmosphere, indicating that the conversion of
Ϫ
methanol into methyl acetate does not involve carbonylation. The effect of the addition of Cl ion on the reaction
of formaldehyde was similar to that when using methanol as substrate, but quite different activation energies were
Ϫ1
Ϫ1
obtained for the formation of methyl acetate (27.9 kJ mol ) and methyl formate (40.6 kJ mol ). The results are
in accord with rate-determining dehydrogenation of methanol to formaldehyde (as a common intermediate),
which is then converted competitively into acetic acid and methyl formate with different activation energies. The
Ϫ
Ϫ
effect of added free Cl ion for both substrates suggests the presence of a pre-equilibrium dissociation of Cl
Ϫ
from the co-ordinated SnCl , which is required for the formation of acetic acid but not for methyl formate. A
3
possible reaction scheme is presented, in which reductive elimination of methyl formate is competitive with that of
acetic acid.
We have previously reported the unprecedented reaction in
which acetic acid (and/or methyl acetate due to rapid esterifica-
tion) is formed from methanol alone with the ruthenium()–
3
Ϫ
tin() complexes [Ru(SnCl ) L] (L = PPh3 1 or MeCN 2)
3
5
2
used as homogeneous catalysts. Comparison of the catalytic
activities among a series of ruthenium() complexes [RuCl2-
{
P(OMe) } ] 3, [RuCl(SnCl ){P(OMe) } ] 4 and [Ru(SnCl ) -
3 4 3 3 4 3 2
2
{P(OMe) } ] 5 for methanol and possible intermediates in the
3
3
overall reaction [formaldehyde (formed as paraformaldehyde)
and methyl formate] revealed that only 5 could catalyse the
conversion of methanol into acetic acid, which also converted
3
paraformaldehyde or methyl formate into acetic acid. Complex
3
showed merely a catalytic activity for the Tischenko-type
dimerisation (2HCHO → HCO Me), and 4 exhibited an
2
intermediate character, being able to catalyse the two reactions
(
2HCHO → HCO Me and HCO Me → MeCO H) but
2
2
2
Fig. 1 Time course for the conversion of methanol (0.025 mol) into
unable to react with methanol. Thus the unique character of
the SnCl3 ligand is obvious.
When complex 1 is used as catalyst we have preliminarily
observed that the selectivity for methyl acetate vs. methyl for-
mate formation can be varied systematically by the amount of
extra Cl ion added. We have now investigated the mechanism
of this unique conversion of methanol in detail with 1 used as
catalyst.
3
Ϫ
Ϫ7
methyl acetate with [Ru(SnCl ) (PPh )] as catalyst (1.00 × 10 mol),
Ϫ
3
3
5
3
nitromethane solvent (1.0 cm ) at 140 ЊC. The reaction was performed
3
in a glass ampoule (7.0 cm ) sealed under vacuum with the solution
frozen by liquid nitrogen
Ϫ
studies, reactions were performed at a lower temperature
(65 ЊC), and initial rates were determined from the initial linear
slopes of the time vs. conversion curves. During this period the
UV/VIS spectrum of the reaction solution exhibited no appre-
ciable change, showing a single peak with λmax = 360 nm;
however this peak changed into a broad peak with a shoulder at
Results
A representative time course for selective conversion of
methanol into methyl acetate with 1 as catalyst is shown in Fig.
1
4
30 nm when the catalytic activity was lowered considerably.
Virtually stoichiometric formation of dihydrogen and methyl
acetate was ensured (within 5% error).
. The turnover number is calculated as the amount of product
(mol) divided by the amount of complex charged (mol). Methyl
formate was formed in trace amount, and methyl acetate
would be formed from acetic acid by rapid esterification with
the methanol present in excess as a substrate. For kinetic
Dependence on the catalyst concentration
1
–5
Ϫ5
The catalyst concentration was varied between 2.50 × 10 and
Ϫ4
Ϫ3
1
.00 × 10 mol dm , with the substrate maintained at 12.5 mol
Ϫ3
dm . The rate dependence on the catalyst concentration (Fig.
2) clearly shows that the rate is first order with respect to the
catalyst concentration.
†
Synthesis of acetic acid from methanol alone by homogeneous metal
1
complex catalyst. Part 3.
Non-SI unit employed: atm = 101 325 Pa.
J. Chem. Soc., Dalton Trans., 1997, Pages 789–794
789

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Fig. 2 Initial rate of formation of methyl acetate as a function of the
catalyst concentration. Methanol 2.5 mol, solvent (nitromethane) 100
3
cm , under N (1 atm), 65 ЊC
2
Ϫ
Fig. 4 Effect of the addition of Cl ion on the conversion of methanol
into methyl acetate (᭺) and methyl formate (᭹) with [Ru(Sn-
3
Ϫ
Ϫ5
Cl ) (PPh )] as catalyst (1.00 × 10 mol). Methanol 2.5 mol, solvent
3
5
3
3
(
nitromethane) 100 cm , under N (1 atm), 65 ЊC. (a) NEt ClO
2 4
Ϫ5 Ϫ3 Ϫ5 Ϫ3
4
7
2
.50 × 10 mol dm , (b) NEt Cl 5.00 × 10 mol dm , NEt ClO
4 4
Ϫ5 Ϫ3
4
.50 × 10 mol dm
Fig. 3 Initial rate of formation of methyl acetate as a function of
3
Ϫ
substrate concentration (methanol). Catalyst, [Ru(SnCl ) (PPh )]
1
,
3
5
3
Ϫ5
3
.00 × 10 mol, solution volume 200 cm (nitromethane solvent),
under N (1 atm), 65 ЊC
2
Dependence on the substrate concentration
The substrate concentration was varied between 0.125 and 12.5
Ϫ3
Ϫ5
mol dm at the constant catalyst concentration of 5.0 × 10
Ϫ3
mol dm . As shown in Fig. 3, the rate of methyl acetate for-
mation showed saturation at high concentrations.
Ϫ
Effect of extra addition of free Cl
Ϫ
Free Cl ion was added as NEt Cl in the concentration range
4
Ϫ5
Ϫ5
Ϫ3
Fig. 5 Dependence of the ratio of the initial rates for the formation of
1
.5 × 10 –3.5 × 10 mol dm , the ionic strength of the solu-
Ϫ
methyl acetate (ν ) and methyl formate (ν ) on the amount of Cl
1
2
tion (I) being kept constant with NEt ClO . Notably, as
3Ϫ
Ϫ5
4
4
added. Catalyst, [Ru(SnCl ) (PPh )] , 1.00 × 10 mol, methanol 2.5
mol, solvent (nitromethane) 100 cm , under N (1 atm), I = 3.75 × 10
3
5
3
Ϫ
3
Ϫ4
revealed in Fig. 4, the addition of free Cl strongly affected the
product selectivity. While methyl acetate is a main product
without NEt Cl [Fig. 4(a)], addition of NEt Cl makes the for-
2
Ϫ3
mol dm , 65 ЊC
4
4
mation of methyl formate predominant with a concomitant
ucts (Fig. 6). The ratio of the associated rates is given as a
function of [PPh3]ad in Fig. 7. When the reciprocal of the total
rate is plotted as a function of [PPh ] , an almost linear
decrease in methyl acetate [Fig. 4(b)]. Fig. 5 shows the effect of
Ϫ
changing the added amount of Cl ion on the relative rate of
3
ad
methyl acetate and methyl formate formation; the formation of
methyl formate became faster than that of methyl acetate at
dependence is obtained although a slight upward deviation
appears at low concentration (Fig. 8).
Ϫ5
Ϫ3
Ϫ
about 3.5 × 10 mol dm ([Cl ] /[Ru] = 0.7). Despite the
ad
0
1
3
observed drastic change in product selectivity, the addition of
Reactions under CO and CO atmospheres
Ϫ
free Cl ion scarcely changed the total rate of formation for
When the reaction was performed under a CO atmosphere (1
atm) the product selectivity (exclusively methyl acetate) was
unchanged, but the rate of methyl acetate formation was
reduced to ca. ¹. Hence the presence of CO molecules in the
methyl acetate and methyl formate throughout the concentra-
tion range examined (cf. Fig. 4).
¯
³
Effect of extra addition of free PPh3
solution diminishes the reaction rate. It was further found that
Ϫ5
13
Interestingly the extra addition of PPh ([PPh ] = 2.5 × 10 –
even when the reaction was performed under a CO atmos-
3
3 ad
Ϫ4
Ϫ3
13
2
.5 × 10 mol dm ) not only affected the product selectivity
phere no C was incorporated into the methyl acetate formed;
1
3
but also lowered the total rate of formation for the two prod-
the mole ratio of methyl acetate formed to CO charged was
7
90
J. Chem. Soc., Dalton Trans., 1997, Pages 789–794

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Fig. 8 Reciprocal of the initial rate of formation of methyl acetate and
methyl formate (total) plotted as a function of the amount of PPh3
added. Conditions as in Fig. 7
Fig. 6 Effect of the addition of PPh on the conversion of methanol
3
into methyl acetate (᭺) and methyl formate (᭹) with [Ru(Sn-
3
Ϫ
Ϫ5
Cl ) (PPh )] as catalyst (1.00 × 10 mol). Methanol 2.5 mol, solvent
(
[
3
5
3
3
nitromethane) 100 cm , under N (1 atm), 65 ЊC. (a) No addendum, (b)
2
Ϫ4 Ϫ3
PPh3]ad = 1.50 × 10 mol dm
Fig. 9 Temperature dependence of the initial rates for the formation
of methyl acetate (a) and methyl formate (b). Catalyst, [Ru(Sn-
Fig. 7 Dependence of the ratio of the initial rates for the formation of
methyl acetate (ν ) and methyl formate (ν ) on the amount of PPh
3
3
Ϫ
Ϫ5
Ϫ
Ϫ5
Cl ) (PPh )] , 1.00 × 10 mol, [Cl ] = 0 (a) and 5.00 × 10 mol
3
5
3
ad
1
2
Ϫ3
Ϫ4
Ϫ3
3
Ϫ
Ϫ5
dm
(b), I = 3.75 × 10
mol dm
,
methanol 2.5 mol, solvent
added. Catalyst, [Ru(SnCl ) (PPh )] , 1.00 × 10 mol, methanol 2.5
3
5
3
3
3
(nitromethane) 100 cm , under N (1 atm)
mol, solvent (nitromethane) 100 cm , under N (1 atm), 65 ЊC
2
2
Ϫ1
efficient = 0.995) an activation energy of 75.7 kJ mol was
obtained.
sufficiently small (3.5%), and the mass spectral patterns, m/z 74
1
2
13
(
Me CO Me) and 75 (Me CO Me), for the methyl acetate
2 2
12 13
When the temperature dependence was examined in the pres-
formed under CO and CO atmospheres commonly showed
Ϫ
Ϫ
ence of added Cl ion ([Cl ] /[Ru] = 1), where methyl formate
ad
0
an intensity ratio [I(m/z 75)/I(m/z 74)] identical to the value
Ϫ1
1
3
was selectively formed, an activation energy of 76.3 kJ mol
(
3.5%) calculated for the natural abundance of C.
was obtained for methyl formate formation [Fig. 9(b), correl-
ation coefficient = 0.998]. Thus the activation energies for
methyl acetate and methyl formate are virtually identical, when
the respective correlation coefficients are taken into account.
Dependence on the partial pressure of H2
The reaction was performed by varying the partial pressure of
H at a total pressure of 1 atm (N + H ). Under these condi-
2
2
2
tions methyl acetate was still formed exclusively at a virtually
constant rate.
Reaction with formaldehyde
It was suggested that formaldehyde was formed in the first step
of conversion of methanol into both acetic acid and methyl
formate as a common intermediate. Therefore we performed
Activation energy
3
The temperature dependence of the rate of methyl acetate
formation is shown in Fig. 9(a) in the form of an Arrhenius
plot. From the slope of the straight line (correlation co-
the reaction of formaldehyde (i.e. paraformaldehyde as a for-
maldehyde precursor) with the present catalyst. Interestingly
Ϫ
the effect of extra addition of free Cl is quite similar to that in
J. Chem. Soc., Dalton Trans., 1997, Pages 789–794
791

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initial dehydrogenation step to produce formaldehyde (either
6
free or co-ordinated ) is rate determining, which would be con-
verted relatively rapidly into either of the two final products.
Since the rate of methanol conversion is independent of the
partial pressure of H , the dehydrogenation step should be
2
irreversible (not an equilibrium), which strongly supports the
postulation above.
Ϫ
As the effect of added free Cl ion on the total rate of
methanol conversion was very small, it is feasible that the rate-
determining step to produce formaldehyde does not include any
Ϫ
equilibrium dissociation of Cl . The processes after the rate-
determining step may competitively produce methyl formate
and acetic acid, and their relative contribution would be
Ϫ
affected by the presence of free Cl ion (see below).
Since the total rate to form the two products is diminished
upon the addition of free PPh (Fig. 6), the presence of a pre-
equilibrium dissociation of PPh3 ligand from the catalyst is
indicated as in equation (1). If the intermediate formed gives
3
Fig. 10 Dependence of the ratio of the initial rates for the formation
Ϫ
of methyl acetate (ν ) and methyl formate (ν ) on the amount of Cl
1
2
3
Ϫ
Ϫ5
added. Catalyst, [Ru(SnCl ) (PPh )] , 1.00 × 10 mol, paraformalde-
hyde equivalent to 1.25 × 10 mol formaldehyde, solvent (nitrometh-
ane) 100 cm , under N (1 atm), I = 7.50 × 10 mol dm , 65 ЊC
3
5
3
Ϫ2
3
Ϫ4
Ϫ3
K
3Ϫ
2
[
Ru(SnCl ) (PPh )] + MeOH
3 5 3
3
Ϫ
[
Ru(SnCl ) (MeOH)] + PPh3 (1)
3 5
the product (formaldehyde) with a first-order rate constant k,
the overall rate can be expressed as in equation (2) where
ν = kK[Ru] [MeOH]/(K[MeOH] + [PPh ])
(2)
0
3
[
Ru] is the concentration of catalyst charged. The linear correl-
0
ation between ν and [Ru] (Fig. 2), and the saturation curve for
0
ν vs. [MeOH] (Fig. 3) are both consistent with this equation.
Equation (2) can be rearranged to give (3). This equation
1
1
[PPh3]
(3)
=
+
ν
k[Ru]0 kK[Ru] [MeOH]
0
accounts for the linear relationship between 1/ν and [PPh3]ad
(Fig. 8). The upward deviation in the low-concentration region
is due to the inherent contribution of ligand dissociation to the
total concentration of the free phosphine, which should become
appreciable in this region. The values of k and K obtained from
Fig. 11 Temperature dependence of the initial rates for the formation
of methyl acetate (᭺) and methyl formate (᭹). Catalyst, [Ru(Sn-
3
Ϫ
Ϫ5
Ϫ
Ϫ4
Cl ) (PPh )] , 1.00 × 10 mol, [Cl ] = 0 (᭺) and 1.50 × 10 mol
3
5
3
ad
Ϫ3
Ϫ4
Ϫ3
dm (᭹), I = 7.50 × 10 mol dm , paraformaldehyde equivalent to
.25 × 10 mol formaldehyde, solvent (nitromethane) 100 cm , under
Ϫ2
3
1
Ϫ1
Ϫ5
the slope and intercept are 0.71 h and 2.2 × 10 , respectively.
N (1 atm)
2
As the extra addition of PPh affected the product selectivity
3
7
(
Figs. 6 and 7), co-ordination of PPh to the tin() site should
3
the case of methanol as substrate; while methyl acetate and
be taken into account (see below). However, since the additive
acetic acid were exclusively formed without added NEt Cl, add-
4
effect is relatively small and requires a large amount of PPh
3
ition of NEt Cl made the formation of methyl formate appre-
4
Ϫ
(in comparison to the case of Cl ), the effect of co-
ciable (Fig. 10). The initial rates for the two products are almost
ordination may be neglected when equation (1) is considered.
It is to be noted that catalytic dehydrogenation of methanol
to produce formaldehyde (+ formaldehyde dimethyl acetal)
and/or methyl formate is possible with simple ruthenium()
Ϫ
the same at [Cl ] /[Ru] = 0.8, and the formation of methyl
ad
0
Ϫ
formate became predominant above [Cl ] /[Ru] = 1.3. From
the temperature dependences of the reaction rate determined
with [Cl ] /[Ru] = 0 and 1.5, activation energies of 27.9 and
0.6 kJ mol were obtained for the formation of methyl acetate
ad
0
Ϫ
ad
Ϫ1
0
Ϫ
8–10
Ϫ
complexes without SnCl3 as ligand.
Therefore, SnCl3
4
would act merely as an auxiliary ligand in this dehydrogen-
ation process.
(+ acetic acid) and methyl formate, respectively (Fig. 11).
Ϫ
It has definitely been shown that extra addition of Cl blocks
Discussion
No incorporation of C into the product methyl acetate in the
the reaction path for acetic acid formation not only for the
methanol substrate (Figs. 4 and 5) but also for the formalde-
hyde substrate (Fig. 10) in a similar way. This fact may also
support the intermediacy of formaldehyde in the overall reac-
tion. This blocking effect can be explained in terms of an equi-
1
3
1
3
experiment with CO evidently indicates that the conversion of
methanol into methyl acetate does not involve any process of
carbonylation. The rate retardation on changing the reaction
atmosphere from N2 to CO is due to the inhibitive co-
ordination of CO to the catalytic site. These results support the
Ϫ
Ϫ
librium dissociation of Cl from the co-ordinated SnCl ,
3
which is required for the formation of acetic acid but not for
methyl formate. Since the dehydrogenation step to form for-
maldehyde is considered to be rate determining, a rather large
difference in the activation energy observed when formaldehyde
is used as substrate (Fig. 11) is associated with the competitive
processes after the rate-determining step. The smaller magni-
tude of these activation energies compared to that for methanol
3
previous postulation that the reaction involves the initial
dehydrogenation of methanol to formaldehyde which is then
converted into methyl formate or acetic acid.
Since (i) the activation energies for methyl acetate and methyl
formate formation from methanol were virtually identical (Fig.
9
), and (ii) the total rate of their formation was hardly changed
Ϫ
Ϫ1
upon extra addition of free Cl ion over a wide concentration
as substrate (≈76 kJ mol ) may be consistent with this view.
range (cf. Fig. 4), it seems reasonable to postulate that the
The higher activation energy for methyl formate formation
7
92
J. Chem. Soc., Dalton Trans., 1997, Pages 789–794

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MeOH
slow
HCHO
Me
Ln Ru
HCO Me
2
O
C
O
H
H
O
Ln Ru
MeCO2H
C
O
Me
Fig. 12 Dependence of c A /c A for methanol (᭺) and paraformalde-
hyde (᭹) substrates [cf. equation (4) in the text] on the amount of Cl
2
2
1
1
Ϫ
added. Reaction conditions as in the captions of Figs. 5 and 10,
respectively
Scheme 1 Competitive formation of methyl formate and acetic acid
from methanol alone via the methyl(formato) and hydridoacetato
complexes, respectively
Ϫ1
(
40.6 kJ mol ) suggests that this process is energetically dis-
Ϫ
favoured, even if it can proceed with Cl retained in the co-
ordinated SnCl . Thus, without the blocking effect of added
Cl ion, methyl acetate should be formed preferably with a
lower activation energy (27.9 kJ mol ) accompanying chloride
Ϫ
14
3
complex. This may be a driving force for the relevant isomeris-
Ϫ
ation process, since the substituent effect on the carboxylate
group (H or Me) would be weaker than the steric (and/or elec-
tronic) effect of the respective ligands.
Ϫ1
dissociation.
Equation (4) may be deduced if the rate is first order with
The observed product selectivity strongly suggests that the
II
Ϫ
vacant site on Sn formed upon releasing a Cl ligand promotes
the isomerisation step in Scheme 1. In view of the bimetallic
nature of the catalytic site, the promotion effect may operate
possibly through the formation of µ-carboxylate-type
ν /ν = [c A exp(Ϫ E /RT)]/[c A exp(Ϫ E /RT)] (4)
2
1
2
2
a
2
1
1
a
1
respect to the concentration of catalytically active species,
1
5
where ν and ν are the rates of formation of methyl acetate and
bridging and/or the stabilisation of a CO -co-ordinated
2
13
1
2
methyl formate, respectively, c is the concentration of the spe-
intermediate.
1
Ϫ
Ϫ
cies lacking Cl , and c that of the species retaining Cl . In the
2
Ϫ
case of ν /ν = 1 ([Cl ] /[Ru] = 0.78, Fig. 10), for instance,
2
1
ad
0
Conclusion
equation (5) is obtained. Substitution with the observed Ea
c A )/(c A ) = exp[Ϫ (E Ϫ E )/RT] (5)
values and T (338.15 K) yields 151 for (c A )/(c A ). Fig. 12
It is demonstrated that the selectivity for the conversion of
(
methanol using complex 1 as catalyst can be controlled system-
2
2
1
1
a
1
a
2
Ϫ
atically by extra addition of free Cl or PPh . Although such
3
addition enhanced the formation of methyl formate relative to
2
2
1
1
(
solid circles) shows a plot of this ratio {calculated from
methyl acetate, the total rate of methanol conversion was not
Ϫ
Ϫ
(ν /ν )exp[Ϫ (E Ϫ E )/RT]} as a function of [Cl ] /[Ru] .
changed with Cl but was reduced considerably by PPh . The
2
1
a
1
a
2
ad
0
3
The open circles correspond to the values estimated using
activation energies for the formation of methyl acetate and
Ϫ
ν values for methanol as substrate and E values for formalde-
methyl formate, obtained under different conditions of [Cl ] ,
a
ad
hyde as substrate. Based on the assumption of the common
intermediacy of formaldehyde, the two plots should be close to
each other. This seems to hold fairly well, and one of the
reasons for the small discrepancy may be the difference in reac-
tion medium [paraformaldehyde dissolved in nitromethane vs.
nitromethane–methanol (1:1, v/v)].
are virtually identical, which suggests the presence of a
common intermediate, the formation of which is rate determin-
ing for the two final products.
1
3
No incorporation of C into methyl acetate in the experi-
1
3
ment with CO indicates the lack of a carbonylation process in
Ϫ
its formation. A similar dependence on [Cl ] for the formation
ad
On a thermodynamic basis, acetic acid is preferred to methyl
of methyl acetate (+ acetic acid) and methyl formate with for-
maldehyde as substrate strongly suggests that formaldehyde is
the presumed intermediate, the formation of which is rate
determining. The invariance of the rate of reaction on the par-
1
1
formate as a dehydrogenation product of methanol, e.g.
at 25 ЊC: 2MeOH(g) → MeCO H(g) + 2H , ∆H Њ = Ϫ32.5 kJ
2
2
Ϫ1
Ϫ1
mol , ∆G Њ = Ϫ51.7 kJ mol ; 2MeOH(g) → HCO Me(g) +
H , ∆H Њ = 52.6 kJ mol , ∆G Њ = 27.8 kJ mol . Therefore, if
2
Ϫ1
Ϫ1
2
tial pressure of H supports this view.
2
2
an appropriate catalyst is provided, acetic acid can be formed
more easily than methyl formate. Further, the ∆G Њ values sug-
gest that a rather high temperature is necessary to obtain a high
equilibrium conversion for methyl formate, but room tem-
perature is enough for acetic acid.
After its rate-determining formation, formaldehyde would
then be converted competitively into methyl acetate and methyl
formate. The former process is energetically favoured (E = 27.9
a
Ϫ1
kJ mol ), but would require pre-equilibrium dissociation of
Ϫ
Ϫ
Cl from the co-ordinated SnCl . The latter process, on the
3
Ϫ1
A possible reaction path is shown in Scheme 1, which repre-
sents the competitive formation of acetic acid and methyl for-
other hand, is energetically difficult (E = 40.6 kJ mol ), but
a
Ϫ
would proceed with retention of Cl . On these grounds, a select-
3
mate after the rate-determining step. This scheme is based on
ivity factor, (c A )/(c A ), was evaluated for both methanol
2
2
1
1
Ϫ
the grounds that a methyl(formato) complex is formed from
formaldehyde, which then gives methyl formate upon reductive
and formaldehyde as substrate with various values of [Cl ]
ad
(c = concentration of catalytically active species, A = pre-
exponential factor for Arrhenius equation).
1
2
elimination, and that the methyl(formato) complex could be
isomerised into a hydridoacetato complex and give acetic acid
Kinetic analysis of the rate retardation upon addition of free
PPh indicates a pre-equilibrium dissociation of PPh ligand in
1
3
upon reductive elimination. Usually a hydride complex is
thermodynamically more stable than the corresponding methyl
3
3
the conversion of methanol into formaldehyde. In this step as
J. Chem. Soc., Dalton Trans., 1997, Pages 789–794
793

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well as in the conversion of formaldehyde into methyl formate,
SnCl3 would simply act as an auxiliary ligand.
Science, Sports and Culture, Japan (Grant-in-Aid for Scientific
Research on Priority Areas, No. 08232221) is gratefully ack-
nowledged.
Ϫ
A possible reaction scheme is presented for the competitive
formation of methyl acetate and methyl formate from for-
II
Ϫ
maldehyde. The vacant site on Sn formed by release of a Cl
References
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bridging and/or stabilise the CO -co-ordinated intermediate to
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2
3
1
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3
Ϫ
The complex [Ru(SnCl ) (PPh )] was synthesized as reported
4 T. Yamakawa, P. Tsai and S. Shinoda, Appl. Catal. A, 1992, 92, L1.
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3
5
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1
6
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1
17.
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dried over CaH and then Na, and distilled immediately before
2
6
use. Nitromethane was dried over CaSO . Carbon-13 monoxide
4
2
1
3
(
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7
8
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[
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mol) and
4
3
3
5
3
3
3
methanol (100 cm , 2.5 mol) in nitromethane solvent (100 cm ).
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2
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1
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1
7
mercury seal was attached. For precise volumetric measure-
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The products were identified by gas chromatography–mass
spectrometry (VG Analytical Autospec-Q) using a 30 m DB-
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1
1
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.
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1
Acknowledgements
Partial financial support from the Ministry of Education,
Received 13th September 1996; Paper 6/06314D
7
94
J. Chem. Soc., Dalton Trans., 1997, Pages 789–794