Solvent induced nitrogen NMR shielding variations of some covalent isocyanates

Journal of

MOLECULAR STRUCTURE Journal of Molecular Structure 328 (1994) 253-257

Solvent induced nitrogen NMR shielding variations of some covalent isocyanates M. Witanowski”, Z. Biedrzyckaa, W. Sicinska”, G.A. Webbby* aInstilute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52,01-224 Warsaw, Poland bDepartment of Chemistry, University of Surrey, Guildford, GU2 5XH, UK

Received 10 June 1994

Abstract Investigations of solvent-induced nitrogen shielding variations for three covalent isocyanates show that the small changes observed are governed mainly by solvent polarity effects. A less significant contribution arises from solvent to solute hydrogen bonding, since the two contributions are of opposite signs, the overall range of solvent induced changes in the nitrogen shieldings of covalent isocyanates is relatively small compared with those of C=N moieties contained in imino-type structures or heteroaromatic rings. Solvaton model nitrogen shielding calculations support the observed signs of the solvent polarity effects exhibited by the nitrogen shieldings of these groups of molecules. Analysis of the observed nitrogen shielding variations with respect to a change of solvent shows that oxygen, rather than nitrogen, is the preferred site for solvent-to-solute hydrogen bonding in the covalent isocyanates.

1. Introduction

We have previously shown that nitrogen NMR chemical shifts (shieldings) provide a deep insight into solute-solvent interactions [l-8]. Nitrogen atoms are usually important sites for such interactions and, using nitrogen NMR, direct access to them is possible. Typically an increase in solvent polarity induces a significant increase in the magnetic shielding of the nitrogen nucleus in > C=N moieties where the nitrogen atom bears lone-pair electrons. However, there appear to be some exceptions to this general observation which include cases from our previous investigations [3-5, 81 where the carbon atom, doubly bonded to nitrogen, belongs to the sp type of hybridisation in the conventional description of bonding orbitals and *Corresponding author. 0022-2860/94/$07.00 0 1994 Elsevier SSDI 0022-2860(94)08375-R

geometries. This includes covalent isothiocyanates, R-N=C=S [8], the carbodiimide structure R-N= C=N-R [4] and covalent azides [5] RN3. In the last case, the central sp hybrid&d atom is nitrogen rather than carbon. Currently we propose to extend our studies to include another member of this group, the covalent isocyanate structure (Fig. 1). As before [l-8] we employ the sign convention whereby a positive sign corresponds to an increase in the magnetic shielding of a nucleus; thus we use the term “nitrogen NMR shielding” rather than “nitrogen chemical shift”, the latter of which uses the opposite sign convention. Apart from the opposite signs, the two terms are equivalent.

2. Experimental Compounds 1 [9] and 2 [lo] were prepared using

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254

h4. Witanowski et al./Journal of Molecular

R = CH3CH2

1

R = (CH3)3C

2

R =

phenyl

3

Fig. 1. Structures of covalent isocyanates studied with a schematic representation of lone pair electrons.

previously published procedures and compound 3 is commercially available. Special attention was paid to the use of very pure and dry solvents as reported previously [8]. All solutions were prepared and handled under a dry argon atmosphere in glove bags. The 14N NMR shielding data were measured at 36.14 MHz on a Bruker AM500 Spectrometer at 35 f 0.2”C, the temperature being maintained by a VT unit. Systematic errors were reduced to less than 0.1 ppm when comparing the nitrogen shieldings of the solute in different solvents. External neat liquid nitromethane was used as a reference using 10 mm/4mm outside diameter coaxial tubes. The inner tube contained 0.3 M nitromethane in acetone-&; the nitrogen shielding

Structure 328 (1994) 253-257

of this solution was +0.77ppm with respect to that of neat nitromethane [l] under conditions where bulk susceptibility difference effects are non-operative (in concentric spherical sample/ reference containers). The value of +0.77ppm was used as a conversion constant. Consequently the contents of the inner tube acted as a precise reference to the neat nitromethane standard and provided a source of deuterium lock for the system. The 14N signal of neat nitromethane has the resonance frequency of 36.141524 MHz; the corresponding frequency for a bare nitrogen nucleus was calculated to be 36.136826 MHz [l 11. This latter value was used in conjunction with the relevant resonance frequency differences in order to obtain the nitrogen NMR shieldings relative to that of the primary standard, neat nitromethane. Lorentzian lineshape fitting of the t4N signals of the samples and external standard was used in order to obtain the relevant resonance frequencies. The nitrogen shielding data so produced were corrected for bulk susceptibility effects as described elsewhere [l 11. Since we used dilute solutions, it was assumed that their susceptibilities are equal to those of the respective solvents at 35°C.

Table 1 Solvent effects on the nitrogen NMR shieldings of some covalent isocyanates, R-N=C=O Nitrogen NMR shielding (ppm) referred to neat nitromethane Solvent

Cyclohexane Et,0 CCl4 Benzene Dioxane Acetone DMSO CHsCls CHCl, EtOH MeOH H20

CFsCH20H Correlation coefficient DMSO, dimethyl suiphoxide.

R = Et

R = phenyl

R=tBu

Measured

Calculated

Measured

Calculated

Measured

Calculated

348.85 348.63 347.68 341.40 346.91 347.04 344.64 346.46 346.30 347.90 _ 344.84 347.80

349.18 348.25 348.21 341.21 346.89 346.25 345.08 346.09 346.39 348.08 _ 345.42 347.34 0.944

325.38 325.39 324.40 324.56 324.00 324.22 322.14 323.84 323.49 324.88 324.86 321.72 324.64

325.61 325.11 325.06 324.50 323.92 323.43 322.56 323.21 323.52 325.05 324.77 322.56 324.09 0.902

333.29 333.66 332.36 332.71 332.44 333.45 330.15 332.43 332.06 334.44 _ 334.81

333.61 333.22 333.02 332.41 332.34 332.08 331.32 332.03 332.40 334.50 _ 334.82 0.852

M. Witanowski et aLlJournal of Molecular Structure 328 (1994) 253-257

255

Table 2 Solvent parameters used and least-squares-fitted solute parameters for a set of master equations (1) Solvent

o

P

R+

6

Dielectric constant W

Cyclohexane Et*0 CCl4 Benzene Dioxane Acetone DMSO CH2C12 CHCl, EtOH MeOH Hz0 CF,CH20H

0 0 0 0 0 0.07 0 0.22 0.34 0.86 0.98 1.13 1.51

0 0.47 0 0.10 0.37 0.48 0.76 0 0 0.77 0.62 0.18 0

0 0.27 0.29 0.59 0.55 0.72 1.00 0.80 0.76 0.54 0.60 1.09 0.73

0 0 0.5 1 0 0 0 0.5 0.5 0 0 0 0

1.87 3.89 2.21 2.25 2.19 19.75 45.80 8.54 4.55 24.20 30.71 76.70

Solute kpm/unit R = Et R=tBu R = phenyl

scale)

1.01 f 0.40 0.81 f 0.39 2.15 % 0.65

b

s

d

(ppm/unit scale)

(ppm/unit scale)

(dimensionless)

0.68 f 0.73 0.99 f 0.71 0.69 f 1.19

-4.61 f 0.64 -3.87 f 0.66 -2.87 f 1.13

-0.16f0.15 -0.26 f 0.18 -0.13 f 0.39

00 (mm)

349.18 f 0.42 325.69 f 0.45 333.67 f 0.66

‘The constants were recalculated for a temperature of 35°C from the data given in [15]. DMSO = dimethyl sulphoxide.

In the case of alcohols as solvents, solvolysis of the isocyanates occurs such that the 14N NMR measurements have to be taken quickly. The sensitivity of the i4N NMR technique is more than adequate to deal with this situation. In such cases, additional i4N signals appear at about +295 ppm from neat nitromethane. These are due to the solvolysis products which are the corresponding carbamate derivatives. Methanol, as solvent, reacts very rapidly with compound 1; the reaction with ethanol is somewhat slower. However, aqueous solutions do not reveal any carbamate signals within about 10min at 35°C which is the typical time required for the 14NNMR measurement. If pure CHC& is used as solvent then the only 14N NMR signal observed is that of the isocyanate. However, in commercial CHCl3, which contains a minute amount of ethanol, an additional signal appears corresponding to the carbamate. The INDO/S SOS shielding calculations, within the framework of the solvaton model [12, 131, were performed on the University of Surrey Primenet System using standard geometries [ 141.

3. Results and discussion The results of high-precision i4N NMR measurements of solvent-induced changes in the nitrogen shieldings of compounds l-3 (Fig. 1) are presented in Table 1. The solvent induced shielding variations are about 4ppm; these are exceeded by shielding changes owing to a variation in the hydrocarbon substituent attached to the NC0 moiety. This is an unusual case since in most > C=N groups, such as those contained in heteroaromatics and imines, the solvent-induced shielding variation can span a range of about 40ppm which is much larger than changes arising from substituent effects [l-3,7]. In order to unravel the various potential contributions to the solvent-induced nitrogen shielding variations of the solutes (compounds l-3), we have employed the empirical scheme of solvent properties which can be expressed [16,17] by the master equation: a(i,j) = Q(i) + u(i>o(j) + b(i)/?(j) + s(i)[rr’(j> + d(i)&)]

(1)

256

M. Witanowski et aLlJournal of Molecular Structure 328 (1994) 253-257

structure

R.

N=C=O

term a, Eq.(l) (jpmlunit scale)

+l to+2

term s, Eq.( 1) (ppmkmit scale)

-3 to -5

Table 3 Solvaton model calculations of medium polarity effects on the nitrogen NMR shieldings of some covalent isocyanates R-N=C=O Calculated variation in the nitrogen shielding (ppm)

R.

N=C= S

R. N&N-

R.

N=C=N- R

-7.4

Dielectric constant (E)

R = Et

R=tBu

R = phenyl

oto+1

0 to -1

2.0 4.0 10.00 20.00 40.00

0 -3.26 -5.13 -5.74 -6.05

0 -2.84 -4.31 -4.83 -4.91

0 (arbitrary) -2.06 -3.21 -3.64 -3.87

-1.7

-3.8

+1.5

Fig. 2. Nitrogen NMR shielding responses for solvent hydrogen bond donor strength (term a) and solvent polarity (term s) for the covalent isocyanates studied and some related structures, covalent isothiocyanates [8] carbodiimides [4], and azides [5].

where i and j represent the solute and solvent, respectively, u is the relevant nitrogen shielding, Q: represents the hydrogen bond donor strength of the solvent, ,Llrepresents its hydrogen bond acceptor strength, X* is its polarity polarizability and 6 is a correction for polychlorinated solvents (S = 0S)and aromatic solvents (6 = 1). The solute terms a,b,s and d represent the corresponding responses of the nitrogen shieldings to a given solvent property. The go parameter is the nitrogen shielding of the solute in the reference state which is approximated by a cyclohexane solution. The solvent parameter set used in the current study is given in Table 2 together with the leastsquares-fitted estimates of the nitrogen shielding responses. The correlation coefficients for the linear relation between the experimental nitrogen shieldings and those retrieved by means of Eq. (1) are given in Table 1. The most important of the solute terms, corresponding to solute-solvent interactions, given in Table 2, is the s term. The b and d terms are insignificantly small and the term a is also rather small. For all three compounds studied the s term is negative in sign which denotes a decrease in the solute nitrogen shielding as the polarity of the solvent increases. This varies with our earlier findings for the nitrogen shieldings of other > C=N- moieties, i.e. those of heteroaromatic systems and imines [l-3, 71, where the s

term has a similar magnitude but is opposite in sign. The negative sign of s for isocyanates is, however, in agreement with those of related structures as shown in Fig. 2. The variation in sign of s for > C=N- moieties depending upon the state of hybridisation (sp or sp*) of the carbon atom is a reflection of opposite trends in the electron-charge redistribution upon changing the polarity of the surrounding medium. Obviously the state of hybridisation is a conventional means of expressing the bond geometries concerned. In the case of the covalent isocyanates, R-N=C=O the existing experimental data show that the -N=C=O moiety is linear, or almost linear, with the R-N-C angle being about 140” for the compounds concerned [18]. In order to substantiate these comments on the electron charge redistribution in the covalent isocyanates due to changes in solvent polarity we have performed some nitrogen shielding calculations as a function of solvent polarity as expressed by its dielectric constant. We have used the INDO/S SOS molecular orbital scheme [12] as the basis of the shielding calculations and have included the effects of the solvent polarity on the shieldings by means of a reaction field as expressed by the solvaton model [ 131.The values of the relevant dielectric constants are given in Table 2, and the results of the solvaton model shielding calculations are reported in Table 3. As shown the solvaton model correctly predicts the direction of change in the nitrogen shielding with variation in solvent polarity. This shows the ability of such calculations to predict accurately the direction of such nitrogen shielding changes since similar calculations accurately

hf. Witanowski et a/./Journal of Molecular Structure 328 (1994) 253-257

demonstrated the corresponding changes for the other > C=N- moieties studied [l-3, 71. We turn now to a consideration of the term a results given in Table 2. This term shows the influence of solvent-to-solute hydrogen bonding in the solute nitrogen shielding. For > C=N- moieties in imino and heteroaromatic compounds, the a term is positive and often very large, up to about 20 ppm [l-3, 71. There are reasons to believe that its magnitude is closely related to the relevant hydrogen bond strength [2]. For the imino and heteroaromatic compounds previously studied the terms a and s are both positive in sign. This accounts for the very large nitrogen shielding ranges due to hydrogen bonding and solvent polarity effects I observed for these compounds. In the case of the covalent isocyanates, compounds 1-3, the term a is positive and rather small (Table 2). Since the corresponding s term is negative, and since hydrogen bond donor solvents are usually polar substances, the effects of solvent polarity and solvent-to-solute hydrogen bonding to nitrogen partially cancel each other. Thus an explanation is provided for the rather small range of nitrogen shielding variations as a function of solvent for the covalent isocyanates. Since the a term is small for the N=C=O group and there is a further potential hydrogen bond acceptor centre at the oxygen atom, it appears that the oxygen, rather than the nitrogen atom, is the preferred site for hydrogen bonding. The size of the R group attached to nitrogen may be responsible for impairing the approach of potential hydrogen bond donating solvent molecules to the nitrogen atom. If this is the case then the replacement of R = Et by R = tBu, in the covalent isocyanates, should produce an increase in the impairment of the approaching solvent molecules. This trend is reproduced by the a values given in Table 2 for these two molecules.

Acknowledgements We gratefully acknowledge support from NATO

251

for a collaborative research grant. Support under the KBN grant number 2-0897-91-01 from the Polish Committee for Research Advancement is also acknowledged.

References [l] M. Witanowski, L. Stefaniak and G.A. Webb, in G.A. Webb (Ed.), Annual Reports on NMR Spectroscopy, Vol. 25, Academic Press, London, 1993, p. 1. [2] M. Witanowski, W. Sicinska, S. Biernat and G.A. Webb, J. Magn. Reson., 91 (1991) 289. [3] M. Witanowski, W. Sicinska and G.A. Webb, Spectrosc. Int. J., 10 (1992) 25. [4] M. Witanowski, W. Sicinska and G.A. Webb, Spectrosc. Int. J., 10 (1992) 31. [5] M. Witanowski, W. Sicinska and G.A. Webb, Spectrosc. Int. J., in press. [6] M. Witanowski, W. Sicinska, Z. Grabowski and G.A. Webb, J. Magn. Reson., Ser. A, 104 (1993) 310. [7] M. Witanowski, W. Sicinska, Z. Biedrzycka and G.A. Webb, J. Magn. Reson., Ser. A, 109 (1994) 177. [8] M. Witanowski, J. Sitkowski, S. Biernat, L.V. Sudha and G.A. Webb, Magn. Reson. Chem., 25 (1987) 725. [9] J. Colucci, Can. J. Res., Sect. B, 23 (1945) 111. [lo] H. Ulrich and A.A.R. Sayigh, Angew. Chem., 78 (1966) 746. [l l] M. Witanowski, L. Stefaniak and G.A. Webb, in G.A. Webb (Ed.), Annual Reports on NMR Spectroscopy, Vol. 18, Academic Press, London, 1986, p. 1. [12] I. Ando and G.A. Webb, Theory of NMR Parameters, Academic Press, London, 1983. [13] G. Klopman, Chem. Phys. Lett., 1 (1967) 200. [14] J.A. Pople and MS. Gordon, J. Am. Chem. Sot., 89 (1967) 4233. [15] R.C. Weast (Ed.), Handbook of Chemistry and Physics, 64th Edn., Chemical Rubber Co., Cleveland, OH, 1984, p. E-49. [16] R.W. Taft, J.L.M. Abboud, M.J. Kamlet and M.H. Abraham, J. Solution Chem., 14 (1985) 153. [17] M.H. Abraham, P.L. Grellier, J.L.M. Abboud, R.M. Doherty and R.W. Taft, Can. J. Chem., 66 (1988) 2673. [18] J.U. Grabow, N. Heineking and W. Stahl, J. Mol. Spectrosc., 154 (1992) 129, and references cited therein.