Deuterium isotope effects on the " 9 ~ n shielding constants and spin-spin coupling constants I in stannane and the stannonium cation 1

KEVIN L. LEIGHTON A N D RODERICK E. WASYLISHEN' Department of Chemistry, Dalhousie University, Halifm, N.S., Canada B3H 4J3

Received December 5, 1986

I KEVIN L. LEIGHTON and RODERICK E. WASYLISHEN. Can. J. Chem. 65, 1469 (1987) Deuterium induced isotope effects on the "9Sn chemical shielding constants have been measured for stannane and the

stannonium cation; they are found to be approximately -0.403 ppm/D and -0.05 ppm/D respectively. The 1'9Sn shifts in the

, i series SnD,,H4-,, (n 5 4) deviate from additivity as predicted by Jameson and Osten. The primary and secondary isotope effects I on Sn-H spin-spin coupling for stannane were obtained and are - 2.8 Hz and - 1.7 Hz respectively. The primary isotope effect i for Sn-H spin-spin coupling for the stannonium cation was found to be - 11.6 + 7 Hz; an accurate value for the secondary

isotope effect on the spin-spin coupling could not be obtained. The derivative of the 'I9sn shielding constant and J (Sn,H) with respect to extensions in the equilibrium bond length have been calculated for stannane.

KEVIN L. LEIGHTON et RODERICK E. WASYLISHEN. Can. J. Chem. 65, 1469 (1987). Pour le stannane et le cation stannonium, on a mesurC les effets isotopiques induits par le deuterium sur les constantes de

blindage chimique du 'I9sn; on a trouvC qu'ils sont respectivement d'environ -0,403 ppm/D et de -0,05 ppm/D. Les dkplacements du '19sn, dans la sCrie des composts SnD,H4-,, (n 5 4), dCvient de 1'additivitC comme prCditC par Jameson et Osten. Pour le stannane, on a dCteminC les effets isotopiques primaire et secondaire sur le couplage spin-spin Sn-H et ces valeurs sont respectivement de -2,8 Hz et - 1,7 Hz. On a trouvC que l'effet isotopique primaire, pour le couplage spin-spin Sn-H du cation stannonium est de - 1 1,6 2 7 Hz; on n'a pas pu obtenir de valeur prtcise pour I'effet isotopique secondaire sur le couplage spin-spin. Dans le cas du stannane, on a calcule ia dCrivCe de la constante de blindage du 'I9sn et du J (Sn,H) par rapport aux extensions dans la longueur de la liaison en Cquilibre.

[Traduit par la revue]

Introduction Isotope effects on chemical shifts have been known for many

years and have been the subject of several reviews (1-4). For an observed nucleus, A, substitution of the directly bonded nucleus, '"X, with a heavier isotope, ""x, causes a shortening of the A-X bond that leads to a greater shielding in the heavier isotopomer. If the one-bond isotope shift is defined as

[ i ] ~ A A ( " ' / ~ X ) = U ~ ( A ' ~ ~ X . . . ) - u ~ ( A ~ ~ ~ ' X . . . )

(m' > m ) = (T - u '

where the prime indicates the heavier isotopomer, the one-bond isotope shift is normally negative (4). In this study we have measured isotope effects on the '19sn shielding constants in stannane, SnD,,H4-,, (n 5 4) and the stannonium cation, S ~ D , H ~ - , , + (n 5 3). Also, we have obtained experimental values of 'J (Sn,H) and 'J (Sn,D) for each isotopomer of these two compounds.

Jameson and Osten have recently made significant contribu- tions to the interpretation of isotope effects on both shielding constants (4-8) and spin-spin coupling constants (9). Our experimental results are discussed in terms of the theoretical models developed by these authors and we briefly outline their results as theypertain to stannane.

Chetnical shielding For a symmetric molecule of the type AX,, , where mean bond

angle deformations have a negligible effect on the isotope shift, the one-bond isotope shift may be written as

[21 'AA('"'/"x) = ( a ~ ~ / a A r ) ~ ~ [ ( A r , , ) - (Ar,,) '] + ... where (Ar,) is the mean bond length displacement of bond n (5). If secondary isotope effects on (Ar) of other bonds caused by isotopic substitution of the A-X bond are neglected, then

' ~ u t h o r to whom correspondence may be addressed.

the one-bond isotope shift may be approximated (6) as

[31 'AA("'/"x) = ( a u A / a A r ~ ~ ) e [ ( A r ~ f n ) - (ArAnI')1

where (ArAnl) and (ArAI7,) are the mean bond displacements of the A-"'X and A-"' X bonds, respectively. Jameson and Osten (7) have shown that the difference in mean bond displacements can be expressed as a product of (Ar,,,) and a ratio of masses; thus eq. [3] becomes

[4] 'AA(""/"'x) = (dUA/dArAx),(ArAIn) [ (m' - m)/mf]

x (1/2)[mAl(mA + m)l

The mass term in this equation indicates that the '"H substitu- tion of SnH4 should provide a good case for the observation of an isotope shift on u ('19Sn), since the fractional change in mass on isotopic substitution is large; (m' - m)/mf is 0.5 and mA/(mA + m) is = 1. Also the magnitude of the electronic factor, ( a ~ ~ / a A r ~ ~ ) ~ , has been found to be related to the chemical shift range of A. Since tin has a relatively large chemical shift range, -2700 ppm (lo), 'ASn('/'H) is expected to be relatively large.

For a symmetric molecule, such as CH4 (1 1) or NH4' (12), the isotope shift has been found to be proportional to the number of isotopically substituted atoms in equivalent positions; several examples are given in ref. 7. Deviations from strict additivity have been noticed in a few cases (4, 12); however, in general the experimental error associated with measuring isotope shifts is larger than any deviation from additivity. If secondary isotope effects on mean bond displacement are not neglected, devia- tions from additivity may be predicted. For a tetrahedral species ADnH4 -, (n 5 4) Jameson and Osten have predicted deviations from additivity to be in the ratio 0:3:4:3:0 (4, 7, 12). In the case of ND,H4-,,+ (n 5 4) the ratio was observed to be 0:2.8:4:2.8:0. Here we present another case, SnD,H4-, (n 5

4), where the deviations from additivity are much greater than the experimental error.

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CAN. 1. CHEM. VOL. 65, 1987

FIG. 1. (a) Tin-1 19 nrnr spectrum of stannane, SnD,,H4-, (n 5 4) at 134.5735 MHz and -50°C. (b) Expansion of the central region of the spectrum in Fig. l a . The arrows indicate the relative chemical shifts of each isotopomer.

Spin-spin coupling constants Recently Jameson and Osten have summarized isotope

effects on spin-spin coupling (9), of which there have been relatively few reports. In many cases the observed effects are comparable to the experimental error. The primary isotope effect on spin-spin coupling involving H is defined (9) as follows:

[51 A ~ " J ( A , ~ I ' H ) InJ(A,D)I(y~/yo) - I1'J(A,H) I This is the change in coupling constant between two nuclei caused by isotopic substitution of one of the nuclei; n indicates the number of bonds separating the two coupled nuclei. The

largest primary isotope effect on 'J has been observed in phosphine (13) where A,' J ( ~ ' P , ~ / ' H ) is + 12 Hz, approxi- mately 6% of the coupling constant. In general, the primary isotope effect is much smaller; for example in methane the primary isotope effect on ' J ( '~c , 21 ' H) is approximately -0.3 Hz, or less than 0.5% of the carbon-proton coupling constant (1 1). In molecules where neither of the coupled nuclei have a lone pair, the primary isotope effect on the reduced coupling constant, 'K(A,X), appears to be negative (9). Here the reduced coupling constant is independent of the magnetogyric ratios of the two coupled nuclei, y A and y x , and is defined by eq. [6].

[61 K(A,X) = 4 . r r 2 J ( ~ , X ) / h y ~ y x

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LEIGHTON AND WASYLISHEN 1471

The rovibrational model of Jameson and Osten (9) predicts the 21'H isotope effect on ' K(A,~"H)) to depend on both an electronic factor (independent of mass) and a dynamic factor, eq. 171,

The terms of eq. [7] that involve the difference in mean bond length and the mean square amplitudes are generally negative (9). For one-bond carbon-proton spin-spin coupling constants both the partial derivatives in eq. [7] are positive (9); thus, as observed, a negative isotope effect is predicted. By analogy, a negative value for A,' K ( S ~ , ~ I ' H ) in stannane is predicted.

Secondary isotope effects on spin-spin coupling constants are often smaller than primary effects and can be of either sign (9). The secondary isotope effect is defined as the change in observed coupling between two nuclei, A and B, caused by isotopic substitution of a neighbour X. The largest secondary isotope effect reported is in hydrogen selenide where 'J(77Se,H) in H2Se is 0.9 Hz larger than the value observed for this same coupling in HDSe (14). The theoretical interpretation of secondary isotope effects on J is more difficult (9) and will not be discussed further here.

Stannane and the stannonium cation have been chosen for this study because the value of ' J(' 1 9 S n , ' ~ ) is large for each, approximately -1930 Hz and -2910 Hz, respectively (15); with such large coupling constants, these two compounds represent a unique opportunity to investigate 1" and 2" isotope effects on spin-spin coupling constants and not be restricted by large experimental error.

Experimental SnD,H4-,, (n 5 4) was prepared (16) from anhydrous SnC14

(BDH Chemicals, 98%) by reduction with a mixture of LiA1H4 (Aldrich, 95%) and LiAIDj (MSD Isotopes, 99%). The mole ratio, hydrogen:deuterium, employed was 3:2; 0.006 mol of SnD,H4-,, were condensed into a 10-mm od thick-walled nmr tube containing 5 mL of degassed CS2 (BDH Chemicals, Spec. Grade) at -196°C; then the tube was sealed under vacuum. The sample tube was stored at liquid nitrogen temperature.

SnD,,H3-,+ (n 5 3) was prepared (15, 17) by the reaction of 0.01 mol of SnD,H4-,, (H:D mole ratio = 2:3) and 4 mL of HS03F (Aldrich), which had been distilled under nitrogen, in a 10-mm 0.d. thick-walled nmr tube. The hydrogen evolved was pumped off periodically. After 2.5 h at -78"C, no further hydrogen was produced and the tube was sealed under vacuum. To avoid decomposition the sample was never warmed above -78°C.

The ' I 9 ~ n nmr spectra were recorded on a Nicolet 360 NB spectrometer (Bo = 8.48 T) using a broad band variable temperature 10-rnm probe. The spectrum of stannane was recorded at -50°C, and that of the stannonium ion at -80°C. The temperature was maintained at k 1 .O°C by a nitrogen gas flow.

Results and discussion Isotope effects on a

(a) Stannane The '19Sn nrnr spectrum of SnD,H4-,, (n 5 4) is shown in

Fig. l a ; in Fig. 1 b, the central region of the spectrum has been expanded and the chemical shifts of each isotopomer have been indicated. Replacement of each 'H by 2H in stannane results in an isotope shift of approximately -0.4 ppm/deuterium; how- ever, as indicated in Table l the isotope shifts are not strictly additive. The deviations from additivity are 0.0:3.0:4.0:2.9:0.0, in excellent agreement with the predictions of Jameson and Osten (4, 7).

TABLE 1. 'I9sn chemical shifts (in ppm) and deviations from additivity for SnD,H4-,, (n 5 4)

Experimental Perfect value additivity Difference Ratio

SnH4 0.0000 0.0000 0.0000 0.00 SnH3D -0.4266 -0.4026 0.0240 3.03 SnH2D2 -0.8369 -0.8052 0.0317 4.00 SnHD3 - 1.2306 - 1.2077 0.0229 2.89 SnD4 -1.6103 -1.6103 0.0000 0.00

Using the model of Jameson and Osten (4-6) one can estimate the partial derivative (duSn/dArsnH),. TO obtain this derivative one must first obtain an estimate of ArSn,H; see eq. [4]. For symmetrical AX, molecules where isotopic substitution takes place at the terminal position (i.e., X) Jameson and Osten (4, 6) have suggested that to a first approximation one can associate (ArA,H) with (ArA,H)vib where the latter term can be estimated using eq. [B].

Here is the reduced mass in atomic units and D is given by eq. [9] where r e

is the equilibrium bond length and a 2 , a 3 , b2, and b3 are the Herschbach-Laurie parameters, which are tabulated in refs. 4 and 18. An accurate value of re in stannane is available from a recent microwave study of three different deuterated stannanes (19).

Using this procedure we find that (ArSn-H) = 17.65 X A and substitution into eq. 4 gives (duSn/dArsnH),

-92 ppm/A. This is to be compared with a value of -38 ppm/A derived for (duC/dArcH), in methane (4, 8).

The magnitude of (duSn/dhrsnH), calculated here for stan- nane is smaller than the value one might intuitively expect. Previously, values of (duA/dArAH), for the first-row hydrides, AH,, have been found to be proportional to (ao3/r3)A of the p orbitals (4, 5). Applying this argument to Group IV hydrides one estimates

Using values of ( ao3 / r3 ) given by Barnes and Smith (20) one would predict the shielding derivative for Sn in SnH4 to be approximately 9 times that for C in CH,. Using the above arguments one predicts 'ASn(21' H) = - 1.6 ppm/D in stan- nane, approximately four times the observed value. Thus the results here suggest that in the case of Sn factors other than (ao3/r3) must play a significant role in accounting for varia- tions in the electronic factor.

Recent theoretical calculations and shielding derivatives for the first- and second-row hydrides by Chesnut and Foley (21) indicate that the relative polarity of the heavy atom with respect to hydrogen plays an important role in determining the behaviour of the heavy atom shielding derivative. If the heavy atom is highly positive, e.g., NaH, the derivative is positive; however, if the central nucleus is highly negative, e.g. , HCl, the derivative is large and negative (21). The derivative observed in this study for stannane appears to be consistent with the systematic change of derivatives reported in ref. 21.

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1472 CAN. 1. CHEM. VOL. 65, 1987

Hz

FIG. 2. Tin-1 19 nrnr spectrum of the stannonium cation, SnD,H3-,,+ (tz 5 3) at 134.613 MHz and -80°C.

TABLE 2. Primary isotope effect on 'J(HZ) in stannane

*The value of ' J ( " ' S ~ , D ) has been multiplied by the ratio, y,/y,, 6.51440.

Previously a 2 " ~ induced isotope shift of - 1.62 ppm had been reported for the tin chemical shift of Bu3SnH (22).

(b) The stannonium cation The ' 1 9 ~ n nmr spectrum of SnD,H3-,,+ ( n 5 3) is much

more symmetrical than that of stannane and is shown in Fig. 2. This indicates that the isotope shift is much smaller; 1A119~n(2"H) was found to be -0.05 + 0.03 ppm/D, com- pared with -0.403 + 0.001 ppm/D obtained for stannane. The 'l9Sn nmr linewidths were 25 Hz, and not all of the lines of the spectrum could be resolved; thus the errors associated with this experiment were much larger than those for stannane. It is not possible to decide if the isotope shifts deviate from additivity because of the uncertainties in the experimental data.

It is possible to obtain a crude estimate of the shielding derivative for SnH3+; since the bond length is not knownowe must assume that it is about 1.7 A. A value of - 10ppm/A is obtained, which indicates the relative insensitivity of the '19Sn nuclear shielding to Sn-H bond extension in the stannonium ion.

TABLE 3. Secondary isotope effect on 'J(Hz) in stannane

Compounds A, 'J("~s~,H)[ 'H] A,'J("~s~,D)['H]

The small isotope shift and shielding derivative observed for the stannonium cation can be explained in terms of the charge argument mentioned above. Consideration of BH,, BH4- and AlH3 , A1H4-oindicates derivatives of +3.5, -27.0 and + 84.2, + 11.6 ppm/A, respectively; thus in going from SnH3+ to SnH4 one expects the derivative to decrease algebraically as observed.

Isotope effects on spin-spin coupling (a) Stantzatze Table 2 lists the observed coupling constants and values of

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1 LEIGHTON AND WASYLISHEN

TABLE 4. Primary isotope effects on 'J in the stannonium ion (Hz)

*The value of 'J("9Sn,D) has been multiplied by the ratio y,/y,, 6.51440.

the primary isotope effect on spin-spin coupling in stannane. ' J ( " ~ s ~ H ) for SnH4 was found to be - 1933.3 + 0.1 Hz, in very good agreement with the value of - 1933 Hz that has been reported (15). A , ' J ( ' ' ~ S ~ ~ / ' H ) was found to be between -2.7 and -3.0 Hz, the average value being -2.8 ? 0.7 Hz, which is approximately 0.15% of the total coupling constant. Assuming that the first term of eq. [7] is much larger than the second, one can obtain an estimate of (aJ/dArsnH),. The value we obtain is -640 HZ/& compared with + 190 HzIA, the value calculated for 'J(C,H) in methane using semiempirical molecular orbital calculations (9,23). To compare these two values it is necessary to use the reduced coupling constant, K, since this elimi- nates the nuclear y factor. The partial derivative of the reduced coupling constant in stannane is found to be 1.43 X

10'' kg m-2 s-2 A - ~ / A compared to a value of 3.5 x lo2' kg mP2 s-2 A - ~ /A in methane, an indication that the C-H coupling in methane is more sensitive to bond extension than is the Sn-H coupling in stannane.

The secondary isotope effect on ' J ( " ~ s ~ , H ) is - 1.7 2 0.2 Hz (see Table 3), about 0.09% of the total coupling constant. Unfortunately there is not enough information to reliably calculate the unknown derivative in eq. [14] of ref. 9 . As expected, the secondary isotope effect on ' J ( " ~ s ~ , D ) is -0.3 ? 0.2 Hz, approximately 116.514 of the isotope effect on 'J("9Sn,H).

(b) The stannonium cation The primary isotope effect on the spin-spin coupling for

the stannonium ion is - 1 1.7 2 7 Hz (see Table 4). From this we obtain an estimate for the coupling derivative, (aJ/aArSnH),, of -2680 HZIA, significantly larger than the value obtained for stannane, an indication that the coupling constant is more sensitive to bond extension. The secondary isotope is smaller than the experimental error and so an estimate of its magnitude would be meaningless.

Since the value of A , ' J ( ~ ~ P ~ / ' H ) is positive in phosphine (13), a positive value of AP1J( ' 1 9 S n ' / ' ~ ) is anticipated for the isoelectronic SnH3- ion.

undergraduate research scholarship (K.L.L.) and for operating and equipment grants (R.E.W.). Also, we thank Professor C. J. Jameson for a preprint of ref. 4 , and a referee for several helpful comments.

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Council of Canada for financial support in the form of a summer i I

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