J. Phys. Chem. 1981, 85,1540-1545
1540
their high boron boundaries. This conclusion is consistent with the rather limited studies of SmBe1“-12 The actual behavior of EuB, is still in doubt. The rather dramatic change in the Gibbs energy of formation suggests, using the general principle proposed by Johnson,13that the number of 4f electrons has changed in going from the condensed element to the boride. Past efforts to explain bonding in these materials have considered only the two 6s and the one 5d electrons, as would be appropriate for the La-B system. This work suggests that the 4f electrons may be more involved beyond lanthanum than previously thought. The entropy of formation of both LaB, and NdB6 is negative at room temperature and each becomes more (10) J. Etourneau, J. P. Mercurio, and R. Naslain, C.R. Acad. Sci. Paris, 275, 273 (1972). (11) T. Tanaka, R. Nishitani, C. Oshima, E. Bannai, and S. Kawai, J . Appl. Phys., 51, 3877 (1980). (12) G. V. Samsonov, L. Ya Markovskii, A. F. Zhigach, and M. G. Valyashko, “Boron, Ita Compounds and Alloys”, Book 2, AEC-tr-5032, Publishing House of the Academy of Sciences of Ukrainian S.S.R., Kiev, 1960. (13) D. A. Johnson, J. Chem. Educ., 57, 475 (1980).
negative as the temperature is increased. Unfortunately, there are no high temperature thermal data available for NdB, against which this behavior can be compared. Summary Chemical activity measurements have shown that NdB6 can exist over a wide composition range at high temperatures. There also appears to be a wide range at room temperature, but the expected changed in lattice parameters, with composition, was not observed. We suggest that the hexaboride may split into NdB, and NdB9,as does the La-B system, but at a much lower temperature. The Gibbs energy of formation of NdB, is significantly more negative than that of La&, which suggests an increasing hexaboride stability across the first part of the rare earth group.
Acknowledgment. Mary Pretzel prepared the X-ray films, Barbara Mueller assisted in taking the data, and Roy David provided analytical services. Terry Wallace, by his interest and support, facilitated this work. This work was performed under the auspices of the US.Department of Energy.
Effect of Substituents on the Nickel-Induced Contact Shifts in Aromatic Amines. Comparison with Spin Delocalization in Phenyl, Benzyl, and Related Radicals Graham R. Underwood,* William C. Clyde, and Mark S. Zltter Department of Chemistry, New York University, Washington Square, New York, New York 10003 (Received: December 3, 1980)
The effect of substituents on the Ni(acac)2-inducedproton and carbon NMR contact shifts has been studied in a series of anilines, pyridines, and heterocycles. It is found that substituents have little effect on the shifts unless bonded directly to nitrogen or separated from it by just one atom. INDO calculations on 2-substituted phenyl radicals suggest that the singly occupied orbital is bent slightly away from the substituent, and the ESR hfsc’s as well as the nickel-induced shifts reflect this distortion. It is concluded that the specificity of Ni(aca& for the nitrogen lone pair, the relative constancy of its induced shifts, and their ready interpretation make this a useful NMR shift reagent for amines.
Introduction Certain compounds, particularly nickel bis(acety1acetonate), N i ( a ~ a c )induce ~, large changes in the NMR chemical shifts of amines. These are of interest for two reasons: (1)they act as unique shift reagents which are nitrogen-lone-pair specific and are largely independent of all other functional groups in the molecule, and (2) because the shifts are almost entirely contact in nature they provide valuable information concerning spin delocalization mechanisms and spin distributions in the corresponding radical analogues.2-10 (1) I. Morishima, T. Yonezawa, and K. Goto, J. Am. Chem. SOC.,92, 6651 (1970). (2) G. R. Underwood and H. S. Friedman, J. Am. Chem. SOC., 96,4089 (1974). (3) K. Fricke and H. Suhr, Ber. Bunsenges. Phys. Chem., 72, 434 (1968). (4) K. Fricke, Tetrahedron Lett., 1237 (1971). (5) J. D. Roberts and D. Doddrell, J. Am. Chem. Soc., 92,6839 (1970). (6) J. A. Happe and R. L. Ward, J. Chem. Phys., 39, 1211 (1963). (7) R. S. Drago and R. E. Cramer, J. Am. Chem. SOC.,92,66 (1970). 92 (8) T. Yonezawa, I. Morishima, and Y. Ohmori, J. Am. Chem. SOC., 1267 (1970). 0022-365418 112085-1540$0 1.2510
For these reasons we were particularly interested in two articles dealing with the @-hydrogen ESR hyperfine splitting constant (hfsc) dependence on substitution in @-substitutedethyl radicals. In the first Kochi and Krusicl’ determined wand @-hydrogenhfsc’s in several @-substituted ethyl radicals (-CH2CH2X).From the unusually low values of these splittings and the reasonable assumption that contributions from the major spin delocalization mechanisms should be independent of substituent, they drew the important conclusion that there was significant bridging in these species. Subsequently, Stock and Wasielewski12presented experimental and theoretical data that indicated that the identity of the @ substituent did indeed affect the efficiency of spin transmission to the @ protons. They found that aoHin para-substituted nitrobenzene radical anions varied (9) I. Morishima, K. Okada, T. Yonezawa, and K. Goto, J. Am. Chem. SOC.,93, 3922 (1971). (10) I. Morishima, K. Okada, M. Ohashi, and T. Yonezawa, Chem. Commun., 33 (1971). (11) P. J. Krusic and J. K. Kochi, J . Am. Chem. SOC.,93,846 (1971). (12) L. M. Stock and G. Wasielewski, J . Am. Chem. SOC.,97, 5620 (1975).
0 1981 American Chemical Society
The Journal of Physical Chemistry, Vol. 85,No. 11, 1981 1541
Nickel-Induced Contact Shifts in Aromatic Amines
TABLE I: Relative Experimental Spin Densities Induced in 4-Substituted Anilines by the Addition of Ni(acac), substituent C-1 c-2 c-3 c-4 H- 2 H-3 other -H - CH,
-100 -100
64.9 f 2.9 61.3 i: 1.7
-37.8 -37.3
i: i:
0.7 0.6
51.9 f 3.0 49.4 i: 1.7
-21.6 t 2.4 -25.6 5 0.4
11.6 5 1.6 10.8 t 1 . 2
-OCH,
-100
75.0
5
4.8
-39.2
*
2.6
59.5
-F
-100
64.0 t 62.7 i: 64.3 t 57 64.8 t 58.6 t
1.1 1.1 1.4
-38.4 5 4.2 - 3 8 . 5 t 0.7 -39.4 t 1.4 -42 -43.9 i: 1.2 -40.2 t 1.7
- c1
-Br -CF,a -CO,Et - Ph
a
-100 -100
-100 -100 -100
1.6 1.3
5
2.4
-30.8
0.4
16.4 5 1.6
49.7 i: 58.0 .t 60.95 48 51.3 t 49.2 i:
0.9 1.0 2.3
-30.0 t 2.4 -13.6 t 2.0 -11.2 + 0.4 -30 -30.4 5 0.8 -28.4 i 0.4
11.6 5 0.8 7.6 5 1.2 8.8 5 0.4 10 13.6 t 0.8 17.2 t 0.4
2.9 0.2
t
CH,,-17.6 i: 0.5; CH,, 30.4 .t. 0.8 OCH,, 3.8;OCH,, 4.4 f 1 . 2
CF,, - 26 CO,Et,-24.8 i: 1.2 G 1 ‘ , - 2 3 . 2 + 0.7; C-2’, 16.1 t 0 . 5 ; G3’, -4.9 + 0.1; C-4’, 6.8 t 0.2
Values determined from only two points.
with the identity of the substituent and calculated that, for the C-X bond coplanar with the spin-containing p orbital, upH in @-substitutedethyl radicals should vary by as much as 75% in the series X = -H, -CH3, -NH,, -F, and -NH3+. Several important aspects of this subject remain unresolved, however. What in fact were the dominant conformations present in the mobile radicals for which the experimental data were obtained? Do the calculations, made without the inconvenience of geometric uncertainty, adequately reflect the real influence of the substituents? If so, is this a general phenomenon, with all substituents affecting spin densities at all positions? In order to provide further information which might aid in answering some of these questions, and to assess the impact of these observations on the utility of nickel-induced shifts in amines, we have determined the shifts induced in a variety of substituted amines of essentially fixed geometry and now report the results of this work. We have chosen three series of amines for study: first, 3- and 4-substituted anilines (analogues of the benzyl radical) to examine the effect of substituents of different electronegativities on this highly delocalized radical while maintaining other factors, particularly geometry, unchanged; second, 3- and 4-substituted pyridines (phenyl radicals), a model localized radical of fixed geometry; and third, certain specific nitrogen heterocycles which provide a unique positioning of the electronegative substituent relative to the spin-containing orbital. Results The results are presented in Tables I-VII. The shifts are expressed as relative apparent spin densities obtained from the least-squares slopes of the lines formed by plotting the contact shifts of each atom for five different concentrations of N i ( a ~ a cvs. ) ~ those contact shifts for a reference atom in the same molecules. This procedure has been reported previously.2 The slopes were multiplied by 100 and, as necessary, corrected for differing nuclear gyromagnetic ratios. The errors reported are the standard deviations from the linear regression analysis. In all cases where complexation might have occurred at an atom other than the anticipated nitrogen, it was shown with appropriate model compounds that no such complexation, in fact, occurred. For instance, for thiazole, it was shown that thiophene in the presence of Ni(acac)2gave no measurable shift. Discussion It is seen from Tables I and I1 that the relative spin densities in the benzyl radical analogues are largely insensitive to the nature of the substituent, and, with only
a few minor exceptions, the relative ordering of the spin densities within a given species remains unchanged over a wide range of substituent type. It is worthy of note that in 4-(trifluoromethyl)-,4-phenyl-,and 4-carboethoxyaniline the spin densities at the very different substituent carbons remain constant and that the 4-methyl carbon is but slightly different. There are some variations particularly for H-2 in these compounds although these are not mirrored by the corresponding changes at C-2. Tables I11 and IV show similar behavior for the phenyl radical analogues-the relative spin densities in the ring are again, by and large, unaffected by the identity of the substituent. In the radical analogues discussed thus far, the substituent has been removed from the “radical-site” nitrogen by three or more bonds. In Table V are listed results for some 2-substituted pyridines. These species bear a much closer relationship to the 2-substituted ethyl radicals than any of the above compounds. In examining this table, one should keep in mind that C-2 and C-6 bear the same geometric relationship to the nitrogen lone pair but differ only with respect to location of the substituent; yet the induced shifts are of opposite sign for these two nuclei. Clearly these shifts are severely perturbed by the presence of the substituent. A smaller, but still pronounced effect is observed for C-3 and C-5. A difficulty with using Ni(acac),-induced shifts is that, although they are predominantly contact in nature, there is some uncertainty as to their origin particularly when the nucleus in question is located close to the nitrogen atom. Thus the presence of a substituent close to the site of complexation may distort the geometry of the complex, leading to anomalous effects which bear no relationship to the electronic structure (e.g., pseudocontact contributions). The fact that the three alkyl substituents (Me, Et, i-Pr) induce similar changes while the bromine acts more like hydrogen suggests that both electronic and steric factors may be important. In order to pursue this question a little further, we have performed INDO-MO calculations on phenyl radicals substituted at (2-2, (2-3, or C-4 with -OH, -NH2, -CH3, -H, -F, and -NH3+. The results of these calculations are given in Table VI. This table indicates that, except for the extremely electronegative -NH3+ substituent, hfsc variations are relatively minor. It is probably significant however that the calculated and experimental data all indicate that electronegative substituents in the 3 position tend to favor larger spin densities for C-3 relative to C-5. For the 2-substituted phenyl radicals the calculations also yield more negative spin densities at C-6 than at C-2. Moreover a detailed examination of the eigenvectors reveals that this is accompanied by a distortion of the singly
1542
The Journal of Physical Chemistty, Vol. 85,No. 1I, 1981
Underwood et al.
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The Journal of Physical Chemistry, Vol. 85, No. 11, 1981 1543
Nickel-Induced Contact Shifts in Aromatic Amines
TABLE IV: Relative Experimental Spin Densities Induced in 4-Substituted Pyridines by the Addition of Ni(acac), substituent G2 c-3 G4 H- 2 H-3 other 82.2 92.5 105.0 75.2 89.5
14.4 8.8 .t 3.2 t 8.0 i 4.2
-25.5 -25.5 -18.6 -19.0 -16.9
t i
-CH,
-25.8 t 1.8 -28.0 t 1.0 -34.2 i 0.2 -31.5 t 1.2 -36.5 t 1.8
-C,H,"
-35.0
t
2.5
115.0 t 10.0
-C(CH,),
-27.8
i
1.0
76.0
-Ph
-34.8
t
4.0
-Br - c1 - CN -CO,Et
i
1.2 1.0 0.2 0.4 0.8
100 100 100 100 100
33.0 i 2.2 27.0 * 0.9 25.2 t 0.2 31.9 r 1.2 41.0 t 2.0
-22.0
i
1.5
100
40.0
t
5.0
-16.7
i
1.0
100
34.9
t
3.9
87.8 t 3.8
-17.8
t
1.2
100
39.4
i
1.5
t i
i
i t
CN, 9.0
0.08 i 0.3 CH,, 12.4 t 0.8; CH,, -9.1 i 0.7 CH,CH,, 10.8 * 0.6; CH,CH,, 9.0 i 0.8; CH,CH,, 12.0 t 1.0 C(CH,),, 5.0 t 0.9; C(CH,),, -3.9 t 0.6 C-l', 8.2 i 0.2; C-2', -3.7 f 0.2; c-3'. 1.8 i 0.0; C-4', 1.8 i 0.2 t
CO,Et, 6.2
2.0
Values d e t e r m i n e d f r o m only f o u r points.
occupied orbital such that its larger lobe is directed away from the 2 substituent as if to minimize some repulsive interaction. When a heteroatom is incorporated directly into the framework of the ring (Tables VI1 and VIII), it is seen that the identity of the heteroatom has an enormous influence on the apparent spin density distribution. Note in particular C-3 of isoxazole, isothiazole, and 1-methylpyrazole where the relative spin densities vary by more than 1order of magnitude. It should be pointed out, of course, that the choice of the reference atom in all these systems is arbitrary, and hence it is not possible to tell from these experiments which spin densities have increased and which have decreased. In Table VIII, however, the relative constancy of the values for C-4, C-5, H-4, and H-5 suggests that the major effect is in fact at C-3 and H-3. It is unfortunate that we were unable to obtain more extensive data on the 3-heterosubstituted azoles since these would approach most closely the substitution pattern of the @-substitutedethyl radicals. We were completely unable to obtain meaningful data for oxazole or for H-2 in 1-methylimidazole because of severe line broadening. However it is abundantly clear in these systems also that the proximity of the heteroatom to nitrogen causes substantial changes in the observed shifts. It is instructive to examine these results in the light of other, rather limited, ESR data relating to substituent effects. For benzyl,13 phenoxyl14 and triphenylmethyl15 radicals the hfsc's are essentially independent of the nature and the location of the substituent. Even for the ethyl radical, with the exception of those results noted above, both the carbon and hydrogen hfsc's are relatively insensitive to substituent.16 This general phenomenon, however, appears to be somewhat dependent upon the charge or charge distribution in the radical since several radical a n i o n ~ ~ ~and J ' J polarized ~ r a d i c a l ~ do ~ ~appear J ~ ~ ~to~exhibit rational changes in hfsc with s~bstituent.'~ In previous studies of nickel-induced shifts, we have found it useful to interpret the data in terms of mechanisms of spin delocalization, but, because of the uncer(13) P. Neta and R. H. Schuler, J. Chem. Phys., 77, 1368 (1973). (14) W. T. Dixon, M. Moghimi, and D. Murphy, J. Chem. SOC.,1713 (1974). (15) J. Sinclair and D. Kivelson, J. Am. Chem. SOC.,90, 5074 (1968). (16) J. C. Scaiano and K. U. Ingold, J. Phys. Chem., 80, 275 (1976). (17) E. G. Janzen, Acc. Chem. Res., 2, 279 (1969). (18) E. T. Strom, J. Am. Chem. SOC.,88, 2065 (1966). (19) H. G. Aurich, A. Lotz, and W. Weiss, Chem. Ber., 106, 2845 (1973).
(20) W. C. Danen, C. T. West, and T. T. Kensler, J.Am. C h e n . SOC., 95, 5716 (1973); R. W. Fessenden, J. Phys. Chem., 71, 74 (1964); D. L. Beveridge and K. Miller, Mol. Phys., 14, 401 (1968).
tainty of the degree to which other factors contributed to these shifts, we have been cautious in extending such an interpretation to atoms bonded directly to the complexation site. However, the changes in the a-carbon shifts in the present study are such that it would be inappropriate to let them pass without comment. It is generally recognized that spin is delocalized from a radical site to an a atom lying in the nodal plane of the spin-containing orbital by spin polarization:21
IA
IB
This results in a negative hfsc for the a atom. Moreover, as the atom moves from the nodal plane, the hfsc becomes less negative and in the extreme apparently changes sign.= This is conveniently interpreted in terms of the resonance structures IIA and IIB. In the 3- and 4-substituted
I1A
pyridines all a-carbon shifts are to higher field, consistent with a dominance of the spin polarization mechanism. In the five membered ring heterocycles the a atoms are pulled further from the nodal plane, and all shifts are to lower field. This can be ascribed to an increased contribution from the mechanism described by IIB. In the 2-substituted pyridines the changes are particulary interesting since, if the nonbonding orbital is bent away from the 2 substituent, this moves carbon-6 closer to the nodal plane and carbon-2 further away. The changes in the shifts are entirely compatible with this interpretati~n.~~
Conclusion The carbon and proton NMR shifts induced in a variety of amines by the addition of nickel bis(acety1acetonate) are largely independent of the nature and the location of (21) H. M. McConnell, J. Chem. Phsy., 24, 764 (1956). (22) R. W. Fessenden and R. H. Schuler, J. Chem. Phys., 71,74 (1964). (23) It might reasonably be asked why the effect of the 2-methyl,
2-ethyl, and 2-isoprop 1 substituents have similar effects. We have pointed out previously Ythat the proton shifts of 2-substituted pyridines appear to require that the substituent orientates itself so as to minimize interaction with the Ni(acac)*. Under such circumstances these three substituents would be expected to have similar effects in the complexed species.
1544
The Journal of Physical Chemistry, Vol. 85, No. 11, 198 1
Underwood et al.
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The Journal of Physical Chemktty, Vol. 85, No. 11, 198 I
Nickel-Induced Contact Shifts in Aromatic Amines T A B L E VII:
Relative ExDerimental SDin Densities Induced in 2-Heteroazoles
1545
bv the Addition of Ni(acac L
heteroatom
C-3
c-4
c-5
H-3
H-4
H- 5
NCH,a S 0
105 t 11 80 t 16 494 f 39
249 t 26 2 9 5 t 58 223 * 8
276 t 28 178t 34 221 i 8
6 2 * 18 91 f 5 36 * 5
60.0 * 9.0 58 t 1.0 61 t 3
(100)
other
NCH,, 31.0
* 3.0
(100) (100)
Values d e t e r m i n e d f r o m only f o u r points. T A B L E VIII:
Relative Experimental Spin Densities I n d u c e d i n 3-Heteroazoles by the Addition of Ni(acac),
heteroatom
C-2
NCH,a S
(100) (100)
a
c-5
c 4
72.5 44.5
f f
18.1 6.0
118.0 i 14.7 147.2 t 7.0
H-2 61.6
t
H-4 2.0
62.0 t 10.0 4 0 . 0 t 1.6
other
H- 5
NCH,, 35.5 28.4
f
t
6.0
1.4
Values determined f r o m o n l y f o u r points.
substituents. Moreover, these shifts are easily rationalized in terms of simply described spin delocalization mechanisms.2 It seems probable that the only occasion in which the shifts will vary significantly with substituent will be when a heteroatom is bonded directly to nitrogen or is separated from it by one atom. Even under these circumstances, however, it appears that variations will be amenable to facile interpretation. This suggests also that the observed variation of upHin @-substitutedethyl radicals11J2is not a general phenomenon but is limited to those cases in which the substituent is in close proximity to the radical site. Nickel bis(acety1acetonate) is therefore a useful shift reagent with a unique selectivity for the nitrogen lone-pair orbital and should be of considerable value in the interpretation and assignment of carbon and proton nmr spectra of amines.24
Experimental Section Equipment. All carbon spectra were obtained on a Varian Associates XL-100 NMR spectrometer equipped with a Nicolet TT-100 FT system. All proton spectra were (24) The logic underlying this procedure is as follows. By an examination of the unshifted C-13NMR spectrum, it is usually possible to identify the resonances of those carbons bonded to, or part of, the major substituents in the molecule. The nickel-induced contact shift of a particular resonance is now shown to be characteristically dependent upon the location (relative to nitrogen) of the nucleus responsible for that resonance. Therefore, the various substituents can each be located relative to the nitrogen lone pair, and in this way a substantial portion of the molecule can be built up. Alternatively if, as is frequently the case, the basic framework of the molecule is known but the location of the substituents is in doubt, an unequivocal structural assignment can be made.
obtained on a Perkin-Elmer R20B NMR spectrometer. Materials. All amines were commercially available and were generally adequately pure as purchased. Amine hydrochlorides were converted to their free bases by the addition of an excess of solid Na2C03to an aqueous solution of the salt. The mixture was extracted with ether, and the extracts were dried over MgS04, filtered, and distilled under reduced pressure. The identity and the purity of the products were monitored by standard spectroscopic techniques. 3-Fluoropyridine was a gift from the Olin Corporation. Nickel bis(acety1acetonate) was obtained from AlfaVentron, dried in an Abderhalden drying pistol at 62 "C, overnight, and stored in a desiccator as the solid or as a 0.07 M solution in CDC1,. In the interpretation of the NMR spectra, most of the peaks could be unambiguously identified in both the shifted and unshifted spectra. In such cases, obtaining the least-squares slope for the shifted peaks was trivial. In some cases, however, when Ni(aca& was added and the peaks were shifted, the assignments became uncertain. A graphical technique was used to circumvent this problem. All that was required was one peak whose assignment remained unambiguous throughout all of the spectra. For each spectrum, then, the unambiguous peak was taken as the X value and all other peaks of that spectrum were plotted as a group of Y values. When the data for successive additions of N i ( a ~ a care ) ~ plotted in this way, it can easily be seen which points form straight lines, and the correct assignment of each peak becomes straightforward. This technique proved especially helpful in the assignment of the substituent ring carbon resonances in 3- and 4-aminobiphenyl where several of the carbons had very similar chemical shifts.
