A calorimetric and carbon-13 NMR spectral analysis of the protonation in asymmetric α,ω-thiadiamines

Spectrochimica Ac~a, Vol. 4OA, No. 3, pp. 303-306,

1984

0584-8539/84 $3.00 + 0.00 Q 1984 Pergamon PressLtd.

Printed in Great Britain.

A calorimetric

and carbon-13 NMR spectral analysis of the protonation in asymmetric a,o-thiadiamines

J. SCHAUBROECK, C. T. HUYSand A. M. GOEMINNE Laboratory for General and Inorganic Chemistry-B, University of Ghent, Krijgslaan 281, B-9000 Ghent, Belgium (Received 12 September 1983)

Abstract-Enthalpies of protonation and ‘%-NMR chemical shifts are used to calculate the fractional protonation at each basic site in a,w-thiadiamines. Results are discussed in terms of the inductive effects

exercised by the central thioether group on both terminal aminofunctions.

INTRODUCTION

As discussed in a previous paper [l] the protonation of asymmetric thiadiamines (abbreviated as n,m-NSN) is complicated by the following tautomeric equilibrium:

n,m-NSN +H /

their

2,2-NSN 2,3-NSN 2,4-NSN 3,3-NSN 3+NSN 4,4-NSN

1.0 M solutions in D20 (used as a field/frequency lock signal) at room temperature on a Bruker X5FH90 spectrometer operating at 22.63 MHz. The spectral conditions for the 13CNMR spectra can be defined as follows: pulse width 9 ps; 4 K points: width 5OOHz; number of accumulations: 8392. All spectra were ‘H decoupled with a model B-SU2 (Bruker) broad band decoupler at 90.003850MHz. No shift of the locking frequency with respect to the external reference (TMS) was found. A Radiometer pHM 64 pH meter equipped with an Ingold HA201 glass electrode was used for all pH measurements (2-12pH range). The pH meter was calibrated at 25°C.

This concept also implies that the protonation percentage for each nitrogen after addition of one equivalent of acid will be proportional to its relative microbasicity. Recently we have developed a calorimetric method[2], that proved to be useful in determining the fractional protonation at each basic site in asymmetric IV-methylsubstituted a,w-thiadiamines. This method has now been applied to some asymmetric primary diamines. Results are compared with those obtained with the 13C-NMR technique. The a,othiadiamines studied are listed in Table 1.

RESULTS Previously determined [ 1, 31 heats of protonation and protonation constants of the a,w-thiadiamines are listed in Table 2. Assignments of the 13C-NMR signals are made by comparison with previously studied linear triamines [S]. Since the effect of protonation is largest on carbon atoms in the /I-position of the protonated

EXPERIMENTAL

Materials. The a+z-thWamines were prepared as previously described [ 1,3]. Purity was ascertained by Gran titrations [4] with standardized nitric acid. NMR and pH Measurements. The ‘“C spectra were run for

2,2-NSN 2,3-NSN 2,4-NSN 3,3-NSN 3,4-NSN 4,4-NSN

and

Abbreviation[ l]

NH,-(CH,),-S-(CH&-NH2 NH2-(CH2)2-S-(CH2)s-NHZ NH,-(CH,),-S-(CH2)4-NH2 NH2-(CH2)a-S-(CH2)s-NH2 NH,-(CH,),-s-(C&),-NH, NH2-(CH2).,-S-(CH2)4-NH2

AH,” n,m-H+NSN+H AH;

Table 2. Protonation

a,o.+thiadiamines abbreviations

Formula

n,m-H + NSN

\

1. The

Table

constants and heats of protonation thiadiamines

of a,o-

L+H+ +LH+ -AH, logK,

LH+ +H+ =LH:+ -AH2 log K2

9.682 10.144 10.441 10.366 10.509 10.730

8.821 9.165 9.249 9.628 9.809 10.047

54.1 56.0 56.9 56.5 57.3 57.6

Values in kJmol_’ at 25°C in 0.5 M KNOs. 303

53.3 54.8 55.1 55.9 56.3 56.9

J. SCHAUBROECK

304

site C&9], only these carbon atoms will be considered. The variation with pH of the /?-carbon atom resonance frequency for the asymmetric a,w-thiadiamines is given in Figs l-3. The labelling scheme of the carbon atoms is shown in the molecular formulae on each figure. Calorimetric

et al.

H2N-CH2-CH2-S-CH2-CH2-Cti2-CH2-NH2 A

6PPmA 3L-

8

C

From the protonation scheme (uide supra), it is seen that the relation between the experimentally determined enthalpies at each protonation stage for asymmetric a,w-thiadiamines and the micro-enthalpies is given by AH, = a(AH:) + (1 - a) (AHY)

(1)

AH, = (1 -a)

(2)

(AH;) + c((AH;)

where AH:, AH; are the micro-enthalpies in the first and second protonation step of the nitrogen at n carbon atoms away from the central sulphide group and where AHr, AH: are the microenthalpies in the first and second protonation step of the nitrogen at m methylene groups away from S; tl is the fraction protonated in the first step at the nitrogen which is n carbon atoms away from the sulphide group. Equations (1) and (2) can be rearranged as follows [2]: AH; -AH, AH:-AH,

_ (1 -CO a

(3)

E

F 6;lpH=12751

./’ .’

B

./ /

~;-__---_--..

method

D

.

:/f

/ 6;lpH

= 12751

t

/

I Fig. 2. Variation

)

I

I

9

I,

PH

of’ jC-NMR chemical shifts in ppm for 2,4NSN as a function of pH.

H2N-CH2-CH2-S-CH2-CH2-CH2-NH2 A

B

C

D

E

6 ppm, 63pHrl2621 H2N-CH2-CH2-CH2 A

33

B

-5.CH2-CH2.CH2

c

D

.CH~.NH~ F

E

G

%pm I 6”,ipH=12

6&~=12.62) -__.

,/;

31 -

941

6;1pH’12941

6" _ 29- -

6m - 27 -

I

d;lpH=4691 I 9

Fig. 1. Variation

of 13C-NMR chemical shifts in ppm for 2,3NSN as a function of pH.

Fig. 3. Variation

I , , PH

/ ,,

of I%-NMR chemical shifts in ppm for 3,4NSN as a function of pH.

305

Protonation in asymmetric a,o-thiadiamines

or AH;:-AH, tl AH: - AH2 = (1.

(4)

Assuming that both amino groups in a,o-thiadiamines have almost no influence on each other, then the microenthalpies AH: and AH: in equation (3) will approximately equal the enthalpies of the symmetric diamine n,n-NSN at the first (AH,) and second (AH,) protonation stage, respectively. For the same reason the micro-enthalpies AHF and AH: may be replaced by the enthalpies AH1 and AHz for the symmetric diamine m,m-NSN. The a values thus obtained from equation (3) and (4), respectively are given in Table 3. NMR method

From the observed shift 6 for a particular carbon after addition of one equivalent of acid, the fractional protonation f of the nitrogen perturbing that carbon can be calculated

where (i) S, is the shift of the considered carbon atom when both nitrogen sites are fully protonated; and (ii) & is the shift of the same carbon atom in the completely deprotonated a,o-thiadiamine. For each of the amino groups in an a,o-thiadiamine, equation (5) can be applied, which will yield a, (1 -a) values (6” - 61) u=m

with the superscripts n and m pointing to the chemical shift of the carbon atom in the /3-position of the nitrogen, which is n or m carbon atoms away from the central sulphur atom. Since after addition of one equivalent of acid to a

solution of an a,o-thiadiamine the pH value is given by pH = i (log K, + log K,), 6” and 6” values can easily be obtained from Figs l-3. Results of our NMR estimates of a and (1 - a) with equations (5a) and (5b) are given in Table 4. DISCUSSION

Comparing Table 4 with Table 3 it may be seen that the results from the NMR method agree well with those obtained from equation (4) of the calorimetric method. However, the calorimetric results obtained from equation (3) for 2,3-NSN and 2,4-NSN deviate rather strongly. This is not surprising since the calculation of the a-values from the latter equation involves the enthalpies of protonation of the symmetric thiadiamine 2,2-NSN. Since in the latter diamine both nitrogens are only five atoms away from each other, mutual influence may be. important. Hence, our above made assumption may not be valid in that case. Neglecting the results of equation (3) the mean avalues are respectively 0.32 (+ 0.02) for 2,3-NSN, 0.25 (+ 0.04) for 2,4-NSN and 0.38 (+ 0.06) for 3,4-NSN. These results are in agreement with the greater basicity of the nitrogen that is the farthest removed from the electron withdrawing thioether group. Indeed, the addition of one equivalent of protons to 2,3-NSN results in the formation of about 68”/;, 2,3-NSN+H and 327; 2,3-NH+SN. This difference in degree of protonation must be more pronounced in 2,4-NSN: 75 “/o2,4-NSN+ H and 25 “/D2,4-N+HSN. The increasing difference in basicity between both nitrogens in going from 3,4-NSN to 2,4-NSN is in the same way reflected in the degree of protonation of the nitrogen that is four carbon atoms away from the thioether group: 62 “i;,in 3,4-NSN and 75 7; in 2,4-NSN.

Aclinowledgements-The authors wish to thank Prof. Dr. G. P. VAN DER KELEN for helpful discussion and F. PERSYNfor recording the NMR spectra.

Table 3. a-Values obtained from enthalpies of protonation 2,4-NSN 2,4-H +NSN 2,4-NSN +H a (1-a)

2,3-NSN 2,3-H +NSN 2,3-NSN +H a (1 -a) Ew (3) Pqn (4)

0.44 0.31

0.56 0.69

0.39 0.28

3,4-NSN 3,4-H +NSN 3,4-NSN+H a (l-4

0.61 0.72

0.33 0.33

0.67 0.67

Table 4. a-Values* obtained from “C chemical shifts (ppm) diamine

2,3-NSN 2,4-NSN 3,4-NSN

6”

S:

27.70 27.66 26.10

33.76 33.85 31.53

31.67 32.10 29.12

a [From Eqn WI 0.34 0.28 0.44

S” L1

S” h

6”

(1 -Co [From

Eqn(WI

26.02 25.41 25.49

31.37 30.84 30.93

27.75 26.55 27.50

*The a-values obtained with equation (5a) and (5b) are consistent within 7 7;.

0.68 0.79 0.63

J. SCHAUBROECK et al.

306

BARONE and P. FERRLJTI,J. them. Sot. Perkin Trans. II

REFERENCES

900 (1980). C. T. HUYS, J. SCHAUBROECK and A. M. GOEMINNE, Thermochim. Acta 63, 201 (1983). [2] C. T. HUYS, A. M. GO&N& and Z. EECKHAUT, Thermochim. Acta 65, 19 (1983). [3] G. G. HERMAN and A. M. GOEMINNE, J. Co-ord. Chem. 7, 75 (1977). [4] C. GRAN, Analyst 77, 661 (1952). [l]

[5]

M. DELFINI, A. L. SEGRE, F. CONTI, R. BARBUCCI, U.

[6] [7]

J. C. BATCHELOR, J. H. PRESTEGARD, R. J. CUSHLEY and N. R. LIPSY, J. Am. them. Sot. 95, 6358 (1973). A. R. QUIST, J. R. LYERLA, I. R. PEAT, J. S. COHEN, W. R. REYNOLD and M. H. FREEDMAN, J. Am. them. Sot. 96,

570 (1974). [8] J. G. BATCHELOR, J. Am. them. Sot. 97, 3410 (1975). [9] J. G. BATCHELOR, J. PEENEY and G. K. ROBERTS, J. Magn. Res. 20, 19 (1975).