The kinetics of competitive consecutive second-order reactions: The two-step hydrolysis of dimethoxo(tetra-p-tolylporphyrinato)tin(IV)

Po/y/dmVol. II, No. 13, pp. 1617-1651, 1992 0277-5387/92 $5.00+.00

Printed in Great Britain 0 1992 Pergamon Press Ltd

THE KINETICS OF COMPETITIVE CONSECUTIVE

SECOND-

ORDER REACTIONS: THE TWO-STEP HYDROLYSIS OF

DIMETHOXO(TETRA-p-TOLYLPORPHYRINATO)TIN(IV)

CHING-CHU

TSAI, YAO-JUNG CHEN

and JYH-HORUNG CHEN*

Department of Chemistry, National Chunghsing University, Taichung,

Taiwan 40227, R.O.C.

and

LIAN-PIN HWANG

Department of Chemistry, National Taiwan University and Institute of Atomic

and Molecular Science, Academia Sinica, Taipei, Taiwan, R.O.C.

(Received 4 November

1991;

accepted 3

1

January

1992)

Abstract-The

hydrolysis of a new compound, dimethoxo(tetra-p-tolylporphyrinato)tin

(IV), Sn(tptp)(OMe)z, was studied by NMR spectroscopy. The use of a limited amount of

water in CDC13 allowed the hydrolysis intermediate,

hydroxomethoxo(tetra-p-tolyl-

porphyrinato)tin(IV),

to be identified. The results show that the hydrolysis is a two-step

competitive consecutive second-order reaction, with an absolute rate constant of the first

step k, = (6.63 kO.66) x lop3 SK’ M-

’

and that of the second step

k2 = (3.55kO.35) x low3

SK’

M-’ at 28k2”C.

‘H NMR

spectroscopy

provides a convenient

means of identifying and quantifying reaction inter-

mediates. Previous workers have reported

the

difficulty in detecting dimethoxo(tetraphenylpor-

phyrinato)tin(IV),

Sn(tpp)(OMe)2 (tpp = 5,10,15,

20_tetraphenylporphyrinato),

by ‘H NMR spec-

troscopy due to its rapid hydrolysis to dihydroxo

(tetraphenylporphyrinato)tin(IV),

Sn (tpp)(OMe),,

unless CDCl, is specially dried. ‘,’ Recently, we

reported

the two-step hydrolysis of dimethoxo

(tetraphenylporphyrinato)tin(IV),

Sn(tpp)(OMe),,

by ‘H NMR

spectroscopy.3

In the previous

paper, 3 the absolute values of two consecutive

rate constants

k,

and

k2

could not be determined

since the precise concentration

of water was not

measured. However, an analysis of data led to

an estimate of 2.3 for the ratio

kl/k2.

The new

compound

described in this paper, dimethoxo

(tetra-p-tolylporphyrinato)tin(IV),

Sn(tptp)(OMe),,

was synthesized and used in the hydrolysis invest-

igation. With the aid of ‘19Sn and ‘17Sn couplings,

*Author

to whom correspondence should be addressed.

the stoichiometry of the species Sn(tptp)(OMe), (B),

hydroxomethoxo(tetra-p-tolylporphyrinato)tin(IV),

Sn(tptp)(OMe)(OH)

(C), and dihydroxo(tetra-

p-tolylporphyrinato)tin(IV),

Sn(tptp)(OH),

(D)

(shown in Fig. l), was determined from the relat-

ive intensities of the ‘H signals of the hydroxy and

methoxy groups. The results indicate that there is

a two-step competitive consecutive second-order

process involved in the hydrolysis of Sn(tptp)

(OMe), to Sn(tptp)(OH),.

With the aid of known

water concentration

determined by proton NMR

intensity, the graphical-integration

time-variable

transformtion

method was used to evaluate the

absolute values for both rate constants.

EXPERIMENTAL

Preparation of

Sn(tptp)(OMe),

Sn(tptp)(OMe),

was prepared by substituting

meso-tetraphenylporphyrinato,

tpp, with

meso-

tetra-(p-tolyl)porphyrin,

tptp, in the preparation of

Sn(tptp)(OMe), as described elsewhere.‘*3,4

1648 CHING-CHU TSAI et al.

X Y

(B) OMe OMe

CC) OMe OH

CD) OH OH

Fig. 1. Structure of Sn(tptp)XY complexes.

NMR sample preparation

Deuterated

chloroform

(99.8% from Aldrich)

was dried with CaCl,. The trace amount of residual

water in CDC13 was quantified

by an NMR

method after the sample solution was prepared. The

sample was prepared by dissolving Sn(tptp)(OMe),

in CDC13 to give a concentration of 2.53 x lo-’ M.

Immediately after the freshly prepared solution was

poured into a 5 mm NMR tube, the tube was sealed

with a plastic cap, wrapped with parafilm and mea-

sured at 28 f 2°C.

NMR spectra

‘H and 13C NMR spectra were recorded at 300

and 75.46 MHz, respectively, on a Varian VXR-

300 spectrometer.

RESULTS AND DISCUSSION

The hydrolysis of Sn(tptp)(OMe)* may be expre-

ssed as a two-step competitive consecutive second-

order reaction.

Let

A = H20, B = Sn(tptp)(OMe)2,

C = Sn(tptp)(OMe)(OH),

D = Sn(tptp)(OH), and

E = MeOH.

The chemical equations

for the

hydrolysis of B are

A+B

-C+E

k’

(step 1)

A+CAD+E

(step2)

(1)

(2)

where k, and k2 are the rate constants for steps 1

and 2, respectively.

For eqs (1) and (2), the pertinent rate equations

are

WI

~

=

-k,[A][B]

dt

g=

k,[A][B] - k,[A][C]

y

= k,[A][C].

From the principle of material balance, they read

Pl+[‘4+[Dl=

Plo+[Clo+Pl,

(6)

and

[Al+Pl+[Cl+Pl+[El

= ~~l,+~~l,+~~l,+~~l,+~~l,

(7)

where the subscripts 0 represent the initial con-

centrations of the respective chemical species.

It is apparent that eqs (3H5) may be readily

converted to equations of first-order type by intro-

duction of the paramete?”

8 = &[H,O]dt.

On

integration, we obtain

[B] = [B]Oe-klB

₍₈₎

[C] = [C], e-*“+A

[B]o(e-kle-e-kzB)

(9)

[D] = [D]o+[C]o(l -e-:2’)

1 -he+

₍₁₀₎

The relationship between 8 and

t

is readily estab-

lished by plotting [H,O] against

t

and integrating

graphically. The data giving [H,O] as a function of

t

are then converted to the 8 basis and subsequently

handled as a first-order process.

NMR spectra of Sn(tptp)(OMe),

at various

stages of hydrolysis are displayed in Figs 2 and 3.

The proton NMR spectrum from the hydrolysis of

Sn(tptp)(OMe),

at reaction time

I =

20 min is

shown in Fig. 2. The peaks corresponding

to the

three compounds ED at various times are shown

in Fig. 3. Figure 3a is the NMR spectrum obtained

3 min after dissolution;

it is dominated by the

Sn(tptp)(OMe), resonance j, with a smaller reson-

ance 1’ for Sn(tptp)(OMe)(OH)

and an almost

negligible Sn(tptp)(OH), resonance m. Figure 3b-

d shows increasing production

of Sn(tptp)(O-

Me)(OH) and Sn(tptp)(OH), with reaction time

t.

The time-dependent

concentrations

of the five

species Sn(tptp)(OMe),,

Sn(tptp)(OMe)(OH),

Sn

(tptp)(OH),, MeOH and Hz0 are shown in Fig. 4.

Kinetics of competitive consecutive second-order reactions f g i d a _i,l b c _e I I h mri A I I I I I I I I 4 6 6 4 2 0 -2 -1 -6 -8 wm

Fig. 2. NMR spectrum for Sn(tptp)(OMe), hydrolysis after a reaction time of 20 mm. The spectral resonances used to measure the concentrations of compounds ED, MeOH and H,O are shown. The proton assignments are : (a) H, (pyrrole) ; (b) phenyl H (ortho) ; (c) phenyl H (me@ ; (d) CHCl, ; (e) methyl group of methanol ; (f) p-CHS ; (g) hydroxy group of water; (h) methanol OH group; (i) tetramethyl silane ; (i) methoxy group of Sn(tptp)(OMe),; (1) methoxy group of Sn(tptp)(OMe)(OH);

(1’) hydroxy group of Sn(tptp)(OMe)(OH) ; (m) hydroxy group of Sn(tptp)(OH),.

The initial build-up and subsequent decay of concentration of 2.53 x lo-’ M. As the reaction

Sn(tptp)(OMe)(OH) are clearly evident for

k2 c kl.

proceeds, the increase of methanol concentration

Further experiments indicate that more than 95% and the decrease of water concentration as expre-

of the initial concentration of Sn(tptp)(OMe)z is ssed in eqs (1) and (2) were monitored by integration consumed within

3

h. Thus the reactions with water of the peaks e and g shown in Fig. 2 and comparison

are essentially irreversible. The initial water con- with the known Sn(tptp)(OMe), concentration at

centration was quantified by the NMR integration peak j. The result is also shown in Fig. 4. The value

method with respect to the known Sn(tptp)(OMe)r of the new time variable, 8, at any time was

I

(a) t=3 min

I’

&#.A

4

(u j (b) t=20 min 1 (e)t=50min 1 kk9Omin _i_

Fig. 3. NMR spectra for Sn(tptp)(OMe)r hydrolysis after reaction times of 3 min (a), 20 mm (b), 50 mm (c) and 90 min (d).

1650 . :Sn(tptp)(OMe)Z . :Sn(tptp)(OMe)(OH) % :Sn(tptp)(OH)z +:n30 n :MeOH 3 30 60 100 t (min)

Fig. 4. A plot showing the concentration of compounds A-E with respect to reaction time. The concentrations of these five compounds are obtained by normalizing with eq. (7). The solid curves

are graphed to guide the eye.

. :Sn(tptp)(OMe)z . :Sn(tptp)(OMe)(OH) x :Sn(tptp)(OH)z 0 I I 0.6 P.0 0.0 16.0 t) ( min MI

Fig. 5. A plot showing the concentration of compounds B-D with respect to 0 (see Fig. 4) at 28 +2”C. The solid curves were obtained by computer-fitting with eqs (8)-(10). This yields k, = (6.63 kO.66) x

1O-3 s-’ M-‘; k, = (3.55f0.35)~ 10-3s-’ M-‘; K= 0.54f0.11.

Table 1. ‘H NMR chemical shifts (6 in ppm>” and ‘H-’ “2’ ” Sn coupling constants (J in Hz) for compounds ED Pyrrole H

Phenyl H Phenyl H

Compound 6 J(Sn-H) (ortho) (meta) p-Me

Sn-OH Sn-OMe 6 J(Sn-H) 6 J(Sn-H) B 9.12 - 8.19 (dd) 7.60 (d) 2.73 - -2.16 70.0 (‘19Sn) 66.8 (’ ’ 'Sn) C D Dh D’ 9.13 - 8.21 7.61 2.73 -7.50 35.6 (’ 19Sn) -2.15 71.4 (‘19Sn) 34.0 (’ ’ 'Sn) 68.2 (’ ’ 'Sn) 9.15 10.6 8.22 (d) 7.62 (d) 2.73 -7.48 37.0 (’ ’ 9Sn) - 35.6 (“‘Sn) 9.13 12.2 8.21 7.62 (d) 2.73 -7.49 36.1 9.14 10.3 8.21 7.61 - - 7.46 (br)

“Chemical shifts in ppm relative to TMS. ’ From ref. 11.

‘From ref. 2. dd = doublet.

Kinetics of competitive consecutive second-order reactions Table 2. 13C NMR chemical shifts (6 in ppm)” for compounds ED

Compound C-a _C-8 C-meso c- 1’ c-2 C-3’ c-4 C(p-Me)’ C(OMe)

B 147.3 132.3 121.6 138.6 135.1 127.6 137.8 21.5 44.6

C 147.0 132.4 121.4 138.6 135.0 127.7 137.9 21.5 44.6

D 146.7 132.6 121.2 138.4 135.0 127.7 137.9 21.5

“Chemical shifts relative to the centre line of the CDCl, triplet at 77.0 ppm.

1651

bp-Me is the methyl group at the para position.

determined by graphical integration of & [H,O] dt

and thereby the relationship between [H,O] and 0

established. The t-dependence for compounds B-D

shown in Fig. 4 was then transformed to &depen- dence as shown in Fig. 5. The solid curves were computer-fitted with eqs (8)--(10) at 28 f 2°C. They give k, = (6.63f0.66) x lop3 s-’ M-‘, k2 = (3.55

*0.35)x 10P3 s-’ M-’ and K = k2/k, = 0.54f

0.11. This K value is comparable with K = 0.43

obtained from the hydrolysis of a similar com-

pound, Sn(tpp)(OMe),.3

The ‘H NMR data for compounds ED are dis-

played in Table 1, together with relevant literature data.2.’ ’ ’ 3C chemical shifts for compounds ED are given in Table 2. 13C shifts were assigned with the aid of published data of a similar compound. 3 In this work a controlled trace amount of water has been involved in the study of the hydrolysis

of Sn(tptp)(OMe), to Sn(tptp)(OH),. A two-step

competitive consecutive second-order process with

Sn(tptp)(OMe)(OH) as an intermediate has been

found. The exact solution for the rate constant of the first step is k, = (6.63kO.66) x lop3

s- ’

M-’ and that of the second step is k2 = (3.55 kO.35)

x lo- 3 s- ’ M- ’ at 28 f 2°C. The graphical-integra-

tion time-variable transformation method developed

in this work may be applied to the study of the kinetics of general competitive consecutive second- order reactions.

Acknowledgement-Financial support from the National Research Council of the R.O.C. under Grant NSC 80- 0208-M-005-13 is gratefully acknowledged. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. REFERENCES

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