Nickel Complexes of Aromatic Amines
The Journal of Physical Chemistry, Vol. 82, No. 10, 1978 1175
Contact Chemical Shifts for the Hydrogen Atoms of Nickel Complexes of Aromatic Amines. Spin Density Distributions in Naphthalene and Anthracene Derivatives. Spin Density Transmission through Carbon, Nitrogen, Oxygen, and Sulfur Atoms' Scott R. King2 and Leon M. Stock' Department of Chemistry, Unlversity of Chicago, Chicago, Illinois 60637 (Received August 1, 1977)
Contact chemical shifts are reported for the hydrogen atoms in the aromatic ligands of the nickel acetylacetonate complexes of 1-and 2-aminonaphthalene, 1-and 2-aminoanthracene, 3- and 4-aminobiphenyl, 2-aminofluorene, 4-aminodiphenylmethane, 4-aminobenzophenone, 4-aminodiphenylamine, 4-aminodiphenyl ether, 4-aminodiphenyl thioether, and other selected compounds. The contact chemical shifts measure the relative effectiveness of the T electron systems in these molecules for the transmission of the spin density introduced through the amino substituent. The results for the naphthalenes and anthracenes are compared and contrasted with predictions based on other empirical and theoreticaI models. The results for the molecules with heteroatoms such as 4-aminodiphenylamine suggest that spin density is transmitted more effectively through the amino group than through the other atoms. Conformational factors can play a major role in the determination of the effectiveness of bridging groups for the transmission of spin density. The results for the molecules with bridging heteroatoms are also compared and contrasted with the results obtained in other studies.
Eaton and his associates investigated the distribution of spin density in aromatic structures by the study of the contact chemical shifts of nickel aminotropone i m i n a t e ~ . ~ T o illustrate, they measured the contact shifts for the hydrogen atoms in 1 to assess the spin density in the C2H5
ArNH, t Ni(C,H,O,), 2 (ArNH,),Ni(C,H,O,),
these processes are rapid the nuclear magnetic resonance signals of the nuclei of the aromatic amine are shifted but the lines are not unduly broadened. The shifts of the nuclei in the nickel complexes of interest in this work a.0 described by the Bloombergen-McConnel1 expression6 (eq 1)where the chemical shift, AHi/H, is measured relative oi = AHi/H= -Ui(Te/YN)gflS(S
1 0 II %H4C 'gH5
1
C6H4iC6H5
0
2
2-naphthalene fragment and in 2 to assess the transmission of spin density through the carbonyl group. For the naphthalene derivatives, they found that the positive spin density decreased in the order C1 > c6 > C3 > C8 and that the negative spin density decreased in the order C5 > C4 > Cy. For the benzophenone derivative, they found that the spin density was not propagated through the bridging carbonyl group. They also studied other aromatic fragments and other bridging groups. Their work provided basic information concerning the manner in which spin density distributions are altered in complex i~ electron systems. We have extended this work through an investigation of the contact chemical shifts of the hydrogen atoms of the nickel acetylacetonate complexes of aromatic amines. This study involves work on the phenyl, 1-and 2-naphthyl, 1-and 2-anthryl, and 3- and 4-biphenylyl, and 2-fluorenyl structures as well as work on the propagation of spin density from one aromatic nucleus to another aromatic nucleus through bridging carbon, nitrogen, oxygen, and sulfur atoms. The approach depends on the favorably rapid ligand exchange and electron relaxation rates (eq A): When
+ 1)/3kT
(1)
to the shift in the uncomplexed ligand, S is the total spin quantum number, and the other symbols have their usual significance. The shift depends directly on ai, the hyperfine coupling constant, which is directly proportional to the spin density, pi, in the p orbital of the adjacent carbon atom as indicated by the familiar McConnell relationship6
(2 1 where Q is a constant. Thus, the p-orbital spin densities can be defined by the sign and magnitude of the contact chemical shifts of the hydrogen atoms to guage the effectiveness of the T electron system for the propagation of charge and spin density to quite remote positions. ai = P i &
Results The freshly purified amine was dissolved in deuteriochloroform containing 0.1% tetramethylsilane to give a 0.5-1.0 M solution. Solubility considerations sometimes limited the concentration of the amine. The proton nuclear magnetic resonance spectrum of each solution was recorded a t 270 MHz. The proton signals were generally assigned without difficulty on the basis of chemical shifts and coupling constants. However, the definitive assignment of certain resonances, for example the resonances of 1- and 2-aminoanthracene, required decoupling experiments, shift reagents, and computer simulation of the spectra. Incremental amounts, 5-10 yL, of a solution of anhydrous nickel acetylacetonate, 0.25-0.50 M, in deuteriochloroform were added to the solution of the amines. The nickel-to-amine ratio was always small ranging from to The formation constant is very large and
0022-3654/78/2082-1175$01.00/0@ 1978 American Chemical Society
1176
The Journal of Physical Chemistry, Vol. 82, No. 10, 1978
S. R. King and L. M. Stock A
6.7 x IO-'
5.4 xIO-'
i
i
4.2 xIO-'
& -
2.9 x IO-'
2.1x10-' 1.7 x lo-'
1.3 x I O-'
8.4~
4.2x i ~ - 3 0 8 PPm
0PPm
4 PPm
Figure 1. The NMR spectra of solutions of 2-aminonaphthalene in chloroform-d with added nickel acetylacetonate.
TABLE I: Contact Chemical Shifts for the Hydrogen Atoms of the Nickel Acetylacetonate Complexes of Aromatic Amines in Chloroform at 21 "C Contact shift Amine Aniline 1-Aminonaphthalene 2-Aminonaphthalene 1-Aminoanthracene 2-Aminoanthracene 2-Aminofluorene
Relative contact chemical shift, u/urefH
H 'Jref 9
PPm 7.7 10.7 10.2 7.5 7.3
HI
Hz 1.00 1.00
1.00 1.00
1.00 1.00
H3
H4
H,
-0.44 -0.35 0.42 -0.28 0.29 0.94
1.05 0.88 -0.32 0.75 -0.25 -0.29
-0.44 0.15 -0.18 0.14 -0.14 0.18
H,
H6
1.00 -0.16 0.39 -0.092 0.16 -0.052
0.17 -0.16 0.13 -0.10 0.16
H*
H,
-0.17 0.31 -0.14 0.17 -0.048
HI0
-0.16 0.41 0.21
0.26 -0.22
TABLE 11: Contact Chemical Shifts for the Hydrogen Atoms of the Nickel Acetylacetonate Complexes of Substituted Anilines in Chloroform at 21 "C Contact shift Amine 3-Aminobiphenyla 4-Aminobiphenyl 4-Aminodiphenylmethane 4-Aminobenzophenone 4-Aminodiphenylamine 4-Aminodiphenyl ether 4-Aminodiphenyl thioether
Relative contact chemical shift, u/orefH
H 'Jref 9
PPm 7.4 8.3 5.2 6.8 9.4 6.7
H3
1.00 1.00 1.00 1.00 1.00 1.00
HZ 1.00 -0.47 -0.46 -0.52 -0.43 -0.45 -0.53
H*' -0.050 0.15 0.011 0.007 0.040 0.024 0.019
Bridge -0.80(CHz)
- 0.40( NH)
H3
a The ui/urefH values are 0.99 for the 4 position, -0.44 for the 5 position, and 1.06 for the 6 position. reliable because the signal is partially obscured by the broadened resonance of H2'. Less than 0.0004.
virtually all the nickel is complexed under these conditions. Dilution shifts are negligible. All the signals were measured relative to internal tetramethylsilane. No corrections were applied for the very small differential susceptibility shift. The experimental results for 2-aminonaphthalene which are typical of this work are shown in Figure 1. The linear relationship between the contact shift for each hydrogen atom resonance and the nickel acetylacetonate to amine ratio was examined by the least-squares method to define the slope. The results for 2-aminonaphthalene are shown in Figure 2. The slope of each line was divided by 2 to yield the contact shift, ci, for each nucleus in ppm. These values of q are not wholly independent of the concentration of the amine. Moreover, these values depend on the differences in the binding interactions between the nitrogen atoms of the different ligands and the nickel atom. To avoid the ambiguities which arise from such uncertainties, one of the hydrogen atoms which is ortho to the amino group was selected as an internal reference. The relative values, q/urefH,are independent of the concentration of the amine over a wide range of concentration. The cri
'
H4 -0.055 0.15 O.OO8b
0.023 -0.073 - 0.006 -0.005 -0.020 -0.015 -0.032
0.000c
0.036 0.005 0.006b
* This value is less
-4001
0
20
40
60
80
Figure 2. The relationship between the chemical shifts of the hydrogen atoms of P-amlnonaphthalene and the nlckel acetylacetonate to amine ratio.
The Journal of Physical Chemistry, Vol. 82,No. 10, 1978
Nickel Complexes of Aromatic Amines
TABLE 111: Calculated Coupling Constants for the Hydrogen Atoms of 1-ana 2-Aminonaphthalene
Cation Radicalsa 1-Aminonaphthalene aH/aH, aH,
G
0.75 -0.15 0.31
7 -1.58 8 -0.35
/
aH/aH,
\105H2N
“ . “ a 4 4
W
f
,I \ O
O
9015 7
H2N,
u/urefH
(calcd) (obsd) aH, G (calcd) (obsd) -6.34 1.00 1.00
1 2 -5.01 1.00 3 1.81 -0.36 4 -6.46 1.29 5 -2.41 0.48
6
Chart I
2-Aminonaphthalene
ul‘refH
1177
0.07
1.00
-0.35
0.88
0.15 -0.16 0.17 -0.17
-1.04
0.16 0.53 -0.08 1.51 -0.23 -3.21 0.51 1.97 -0.31 -3.11 0.49
0.42 -0.32 -0.18
0.39 -0.16 0.31
a The calculations were performed for pyramidal geometry at the nitrogen atom.
values for the ortho hydrogen atoms and the relative contact chemical shifts for the other hydrogen atoms in the molecules examined in this work are summarized in Tables I and 11.
Discussion The contact chemical shifts of the aromatic hydrogen atoms could be measured with considerable precision at high magnetic field. The relative values of a/urefH,Tables I and 11, are independent of the concentration of the amine. However, the absolute u values depend on the concentration of the amine and are sensitive to adventitious impurities in the solvent. To circumvent such problems we measured the shifts for all the reference hydrogen atoms, those ortho to the amino groups, under the same condition^.^^ The contact shifts for these atoms, Tables I and 11, indicate that there are no major variations in the equilibrium constants for complexation or in the extent of spin delocalization to the reference hydrogen The relative values of the contact chemical shifts for the molecules in Table I are in excellent agreement with the corresponding values obtained by Eaton and his associa t e ~ For . ~ example, least-squares analyses of the data for 1- and 2-aminonaphthalene and for the related naphthalene derivatives of the tropone iminates, 1, yield correlation coefficients in excess of 0.99 in each instance. Our interest in the contact chemical shifts arose from the idea that the complexed amino group is an electron deficient substituent which strongly interacts with the P electron system of the aromatic molecule. The shifts, therefore, represent a direct measure of the effectiveness of the molecular framework for the propagation of the substituent effect. To the extent that charge distributions are related to spin distributions, these measurements provide a convenient experimental method to examine current proposals concerning the propagation of substituent effects in T electron systems. We first analyzed the results for the naphthylene derivatives using the INDO approach developed by Pople and his associate^.^ This approach has been used to estimate the relative contact shifts for the nuclei of the ligands in the nickel acetylacetonate complexes of aromatic The metal atom is usually neglected in these treatments. For example, Morishima and his associates examined the benzyl radical as a model for the nickelaniline complex.8 We previously examined the aniline cation radical with pyramidal geometry at the nitrogen atom as a model for this complex.4c The ratios of the calculated hyperfine constants for the hydrogen and carbon atoms of this cation radical are in fair agreement with the ratios of the experimental contact shifts for the nickelaniline complex. We have extended this model to the 1-
Chart 11. Values of u/urefH for the Contact Chemical Shifts and the Values of q/qrefc for the n Electron Charge Calculated by the HMO Method (The Charge Ratios Are in Parentheses) (0 25)O l
7
d
l OO(100) (0 100 l
3
d
/
l OO(100)
/
0 I5 0 8 8 ( 0 25)(1 0 0 )
0 14 0 2 6 0 7 5 (0 I I) ( 0 44)(1 0 0 )
(0 25)(1 0 0 ) 031 100
(0 I I) (0 4 4 ) ( I 0 0 ) 017 0 4 1 I O 0
and 2-aminonaphthalene derivatives. The hyperfine coupling constants calculated by this method are summarized in Table 111. It is evident that the model cannot be regarded as satisfactory in the absence of a serious consideration of the orbitals of the nickel atom.4c Nevertheless, with certain exceptions, the calculated coupling constant ratios are in accord with the experimental results. Both the positive and negative ratios are estimated with reasonable accuracy. The experimental results, u / uIefH, for the conjugated positions of the phenyl, biphenylyl, naphthyl, and anthryl fragments yield a consistent series of results. To illustrate, it is striking that the contact shifts for the 2’, 3’, and 4’ hydrogen atoms of 3-aminobiphenyl are reduced in magnitude and opposite in sign from the shifts observed for the 4-aminobiphenyl (Chart I). The distribution of spin density in the substituent phenyl ring reflects the fact that the spin density at the 3 position is negative and about 50% less in magnitude than the spin density at the 4 position. The pattern of the spin density distribution in the second ring is identical with that in the second ring of 4-aminobiphenyl. We conclude that the spin density in the substituent ring depends primarily upon the sign and magnitude of the spin density at the position to which the substituent is bonded and does not depend upon secondary interactions involving the cr framework or other long-range interaction^.^ The observations for the naphthyl- and anthrylamines are in accord with accepted theory. There is appreciable positive spin density a t the 2, 4, 5, and 7 positions of 1-aminonaphthalene, and 1,3, 6, and 8 positions of 2-aminonaphthalene, and the 2, 4, 5 , 7 , and 10 positions of 1-aminoanthracene, and the 1,3,6, 8, and 9 positions of 2-aminoanthracene. Because the complexed amino group is electron deficient, we compared the experimental results for the conjugated positions with the charge densities calculated for the carbon atoms by the HMO method for the corresponding arylmethyl carbonium ions.1° The results shown in Chart I1 indicate that there is general agreement between the values of u/aIefHand
1178
The Journal of Physical Chemistry, Vol. 82, No.
S.R. King and L. M. Stock
IO, 1978
Chart 111. (A) Values of p~ for the Substituent Chemical Shifts for Fluorobenzene and the F1uoronaphthalenes;"g (B) Contact Chemical Shifts for the Nickel Acetylacetonate Complexes of Aniline and the Naphthyl Amines; (C) 71 Electron Charge Densities Calculated for Benzaldehyde and the Naphthaldehydes1lc9 l 2
TABLE IV: Relative Contact Chemical Shifts for Nickel Acetylacetonate Complexes of Aniline Derivatives, Spin Densities for Tropone iminates, and Reaction Constants for Fluorine Chemical Shifts Rel. contact shifta
A subst
Compound Aniline 4-Aminobiphenyl 4-Aminodiphenylmethane 4-Aminobenzophenone 4-Aminodiphenylamine 4-Aminodiphenyl ether 4-Aminodiphenyl thioether
subst
E.
"2
I05
-018 -032
a This study. the 4 position.
L.
CHO
I -0023 \ 4-0081 t 0 0 2 6 /
0 - 0 0 0 9
CHO
I
- o o-0001 i a m-0007 : l l o
-0017 t 0 0 5 0
q / q r e F . This agreement supports the idea that the spin and charge density distributions are related for the conjugated positions and that contact shift measurements provide insight concerning the distribution of charge density as well as spin density. This feature of the results was examined in another way by comparison of the shift data with the empirical reaction constants, ,OR, used to describe the efficiency of the K electron systems in aromatic molecules for the propagation of substituent effects. Several research groups have investigated the transmission of substituent effects in aromatic molecules other than benzene. The work on naphthalene is much more advanced than that on other structures.ll Most recently, Adcock and his students examined the fluorine NMR substituent chemical shifts (SCS) of derivatives of 1- and 2-fluoronaphthalene.llg They adopted the approach developed by Taftlld and used eq 3 to obtain the pI and pR values for the fluoro-
S C S = P I O I+ ~
R u R
U4'-H/U,-H
P4'
PR
1.03d 0.15 0.008
0.0107d 0.00123
-30.5d -3.24 -1.18
0.000
0.0
- 2.74
0.036
0.00146
-8.85
0.0 0 5
0.0002
- 5.07 - 5.90
0.006
Reference 3. Reference 14a. For For dimethylformamide solution.
-, , ^
\ to104 t 0 0 0 3 /
- 3 o o i a - o o i a
to 041
t0.007 $0.042
e
Spin Reaction densityb constant,c*e
(3)
naphthalenes. These pR values are compared with the contact chemical shift data and the K electron charge densities for the related aldehydes in Chart 111. The results obtained in these three studies, Chart 111, are rather closely related especially for the positions which are conjugated with the substituent group. Thus, PR exhibits the largest magnitude for the 4 position in the 1-fluoronaphthalene series (401)and for the 6 position in the 2-fluoronaphthalene series (60). The relative contact chemical shifts and the calculated charge densities are in the same order. In addition, the pR values for the positions with positive spin and charge density are also correlated with 4a > 80 > 5a for the 1-naphthalene derivatives and 60 > 701 for the 2-naphthalene derivatives. In contrast, the pR values, charge densities, and contact shifts are not well correlated for the positions which have negative v/vrefH ~a1ues.l~ This point is illustrated by the finding that v/urefHis negative for the 3 position of the 1 amine and for the 4 position of the 2 amine. These experimental results and the calculated charge densities suggest that p~ should be positive for the 40 and 3a series. The pR value for the 40 series is positive. However, p R for the 3p series is negative. It is pertinent that the empirical analyses
based on eq 3 indicate that pR is, in general, negative for the unconjugated positions whereas computed charge densities and the contact shift work, in general, imply that pR should be positive if this parameter exclusively reflects K electron interactions. This is a troublesome discrepancy. Some portion of the discrepancy may originate in the fact that the linear free-energy relationships for these naphthalene fluorine substituent chemical shifts are imprecisellg and that the small pR values may not be well defined. Adcock and his co-workers have discussed this problem and have noted that several factors could be responsible for the deviations.'lg Nevertheless, it is difficult to reconcile the results and to maintain that the pR parameters reflect only K electron interactions. In summary, the theoretical and empirical analyses suggest that the contact shift data for these aromatic fragments provide a reasonable measure of the relative distribution of charge density at the conjugated positions. However, the contact shift data cannot be used reliably to estimate the relative charge density at the unconjugated positions. The relative contact chemical shifts for the hydrogen atoms in the 4' positions of the molecules (3) with bridging
0x
-
e
N HZ
3
groups are compared with the spin density at the 4' position of the related tropone iminates, 2,3 and with the pR values for fluorine NMR substituent chemical shifts assessed by Taft and his students14cin Table IV. The contact shifts of the amines indicate that the spin density at the 4' position of biphenyl is about 10-fold less than a t the 4 position of aniline. The spin densities for the corresponding tropone iminates, 2, and the reaction constants for the fluorine NMR substituent chemical shifts for the 4-substituted fluorobenzenes and the 4'-substituted-4-fluorobiphenyls also differ by a factor of 10. These observations suggest that conjugative 7r electron interactions between the 4 and 4' positions in biphenyl are about tenfold less important than between the 1 and 4 position in benzene. The contact shifts for the hydrogen atoms in the 4' position of the other amines with bridging groups are all very much smaller than the shift for the 4' hydrogen atom of biphenyl. The shift for the 4' hydrogen atom of the diphenylamine is about fivefold less than for the 4' atom of the biphenyl derivative. The contact shifts for hydrogen
The Journal of Physical Chemistry, Vol. 82,No. 70, 7978 1179
Nickel Complexes of Aromatic Amines
TABLE V: Chemical Shifts for the Hydrogen Atoms of the Amines Compound
1
1-Aminonaphthalene 2-Aminonaphthalene 1-Aminoanthracene 2-Aminoanthracene 2 -Aminofluorene
2
3
4
7
8
9
10
6.69
7.25 6.87 7.28 7.00 6.66 3
7.28 7.61 7.50 7.87 7.61 2
7.76 7.66 7.99 7.91 7.68 Bridge
7.41 6.89 7.46 7.37 7.28 2'
7.39 7.20 7.44 7.37 7.17 3'
7.73 7.55 7.96 7.91 7.45 4'
8.37 8.11 3.77
8.37 8.28
6.59 6.67 6.62 6.67 6.67 6.67
6.96 7.72 6.94 6.87 7.31 7.63 6.84
3.86
7.17 7.72 6.84 6.92 7.11 7.23 7.53
7.25 7.45 7.17 7.27 7.21 7.36 7.38
7.14 7.54 6.77 7.01 7.09 7.36 7.29
6.89 6.73 7.08 6.82
4-Aminodiphenylmethane 4-Aminobenzophenone 4-Aminodiphenylamine 4-Aminodiphenyl ether 4-Aminodiphenyl thioether 4-Aminobiphenyl 3-Aminobiphenyla a
Chemical shift, 6 5 6
5.41
Other 6 values are at 6.61 for H,, 7.18 for H,, and 6.96 for H,.
atoms in the 4' positions of diphenylmethane, diphenyl ether,14 and diphenyl thioether are even less and no detectable contact shift could be observed for the benzophenone. These results suggest that spin density is transmitted more effectively between the rings of biphenyl than between the rings of the other bridged derivatives with the nitrogen linkage more effective than the essentially equivalent methylene, oxygen, and sulfur groups. The carbonyl group is a virtual insulator. The same order of effectiveness was observed for the heteroatoms in the earlier s t ~ d y However, .~ there is an interesting difference between the results of the two studies inasmuch as the data for the tropone iminates suggest that the diphenylamine structure is more effective for the propagation of spin density than the biphenyl structure. In an attempt to resolve this difference, we compared the shifts in planar and nonplanar molecules. The related methyl and methylene groups of 4-aminoacetophenone (4) - 0loo&o 49 \
o//c
0.67
\
CH, -010
-047
4
5
and 5-aminoindanone (5) experience appreciable contact chemical shifts. The observation that the contact shift for the methylene hydrogen atoms of the indanone is about fivefold greater than for the related hydrogen atoms of the acetophenone reveals that enforced planarity has an important influence on the transmission of spin density from the complexed amino group to groups bonded to the carbonyl linkage.15 This point was examined further by a study of the contact shifts for 3-aminofluorenone (6). "2
I
I O O A O 98
-0.44%
-0 12
6
The results for this molecule also indicate that enforced planarity has a major influence on the propagation of spin density. We conclude that the negligible contact shifts observed for the aminobenzophenone and the tropone
iminate, 2, are the consequence of the displacement of the components of the structure from planarity15 rather than from an insulating characteristic of the carbonyl group. The observations for the diphenylamine (7) and the "2
"2
1
0.040
-0.40 " y y O , O E O
W
7
O 036
H / L A - 0 , 0 4
-0 0 0
02
004
8
carbazole (8) provide an interesting contrast. These shifts suggest that there is no more important propagation of spin information in the planar molecule. The results for biphenyl and fluorene represent an intermediate case with only a modest increase in CT7.H/0,,fH for fluorene compared to CT4,-H/q,,fH for biphenyl, Table I. The increase from 0.15 to 0.17, about 15%, is in accord with the idea that the average dihedral angle, ( e ) , between the planes of the benzene rings is about 25" in biphenyl.18J9 The experimental results for the carbonyl compounds and for the biphenyl derivativeslg indicate that conformational preferences can have a major impact on the delocalization of spin density in these molecules. Clearly, more work with rigid molecules will be required for a secure experimental evaluation of the relative efficiency of transmission through heteroatom bridging groups. There is no apparent relationship between the contact shift data for the molecules with heteroatoms and the resonance reaction constants, pR, evaluated by Taft and his associates.14aTheir data imply that the diphenylamine structure is the most effective one for the propagation of substituent effects. The ether and thioether linkages are more effective than the carbonyl linkage and the methylene bridge acts as a near insulator. This order of effectiveness does not correspond with either the order obtained in this study or with the results obtained by EatonS3 Certain differences may be resolved when the stereochemical relationships between the aromatic rings are more fully defined. Fukunaga and Taft recently reported that conformational factors had a major influence on the p1 and pR parameters for shifts in both the boron trichloride complexed and the uncomplexed forms of benzophenone (9) and its tetramethyl derivative ( Steric effects may account for the differences in transmission efficiency. However, there appear to be novel modes for the transmission of substituent effects via the
1180
The Journal of Physical Chemistry, Vol. 82, No. 10, 1978 CH3
9
I CHS
.CH3
\
CH3
10 = 1.82 = 1.50
uncomplexed: P I = 2.55 P I = 2.74
PI p~
BCl, complex: P I = 7.86 p ~ =+ 7.23
P I = 1.82 p ~ =+ 1.26
heteroatom. Taft and his group have proposed that the polarization of the lone pair electrons on the heteroatom results in a conjugative transmission of the polar effect.14* They note that the transmission of polar effects as well as resonance effects occur primarily through the a framework in these molecules. Such interactions could account for the variance of the pR values from the contact shift values inasmuch as rather the special charge polarization effects could alter the charge distribution but not the spin d i ~ t r i b u t i 0 n . l ~ ~ In conclusion, the results of this investigation reveal that the contact chemical shift measurements provide insight concerning the spin density distribution in structures such as naphthalene and anthracene. The spin density at the conjugated positions of these aromatic molecules is apparently closely related to the charge density. Measurements of the spin distribution in other polycyclic aromatic compounds and in heterocycles should prove valuable for the experimental assessment of a electron interactions. The contact shift measurements also yield new information concerning the spin distribution in molecules with bridging heteroatoms. The results establish that enforced planarity can play an important role in the enhancement of spin delocalization. A complete assessment of the relative effectiveness of the different bridging groups will require the study of other model compounds such as aminobenzothiophene. The shift data for these molecules are not related to the pR values derived from NMR experiments because polar effects may influence pR.
Experimental Section Materials. The compounds used in this study were obtained for the most part from commercial sources. 3-Aminobiphenyl, 3-aminofluorenone, and 4-aminodiphenyl thioether were prepared by the reduction of the corresponding nitro compounds using methods described p r e v i o u ~ l y . All ~ ~ the compounds were recrystallized or sublimed prior to use. These compounds exhibited melting points in agreement with the accepted values. In each instance the NMR spectrum showed that the compound was free of contamination. The observed chemical shifts are presented in Table V. Contact Shift Measurements. Nickel acetylacetonate was dried in vacuo at 55 "C for 3 days. Aliquots of a solution of the nickel salt, about 0.25 M, were added to a solution of the amine, about 0.75 M, in deuteriochloroform. The concentration of the nickel reagent was adjusted when the amine concentration was limited by solubility. Tetramethylsilane was used as an internal reference in all the experiments. The proton NMR spectra were recorded following each incremental addition of the nickel reagent, Figure 1. All the spectra were recorded at 270 MHz at the probe temperature, 21 "C. Control experiments were performed to establish that the nickel reagent was complexed preferentially at the primary amino groups in the ethers, ketones, and amines. For example, the addition
S . R. King and
L. M. Stock
of nickel acetylacetonate to an equimolar mixture of aniline and diphenylamine showed that there was no measurable shift for the resonances of diphenylamine while the ortho resonance of aniline shifted by more than 200 Hz.
References and Notes (1) This research was sponsored by the National Science Foundation Undergraduate Research Program and by the Block Fund of the University of Chicago. The magnetic resonance equipment used in this study was made available in part through grants from the National Science Foundation (GP 331 16)and the University of Chicago Cancer Center Grant (Ca-14599-01). (2)National Science Foundation Undergraduate Research Participant. (3) (a) D. R. Eaton, A. D. Josey, W. D. Phillips, and R. E. Benson, Discuss. Faraday Soc., 34, 77 (1962);(b) D. R. Eaton, A. D. Josey, R. E. Benson, W. D. Phillips, and T. L. Cairns, J. Am. Chem. Soc., 84, 4100 (1962);(c) D. R. Eaton, A. D. Josey, W. D. Phillips, and R. E. Benson, J . Chem. Phys., 37, 347 (1962);(d) D. R. Eaton and W. D. Phillips, Adv. Magn. Reson., 1, 103 (1965). (4) (a) E. de Boer and H. Van Willigen, Prog. Nucl. Magn. Reson. Spectrosc., 2, 1 1 1 (1967);(b) G. N. LaMar, W. Dew. Horrocks, Jr., and R. H. Holm, "NMR of Paramagnetic Molecules", Academic Press, New York, N.Y., 1973; (c) L. M. Stock and M. R. Wasielewski, J . Am. Chem. Soc., 99, 50 (1977). (5) (a) H. M. McConnell and R. E. Robertson, J . Chem. Phys., 29, 1361 (1958); (b) J. P. Jesson, ibid., 47,582 (1967). (6) (a) H. M. McConnell, J . Chem. Phys., 24, 632 (1956); (b) H. M. McConnell and D. B. Chesnut, ibid., 28, 107 (1958). (7) J. A. Pople and D. L. Beveridge, "Approximate Molecular Orbital Theory", McGraw-Hill, New York, N.Y., 1970. (8) I. Morishima, T. Yonezawa, and K. Goto, J. Am. Chem. Soc., 92,
6651 (1970). (9) I n some instances, these less important interactions become determinant. For example, the small contact shifts for the 2' and 4' hydrogen atoms in aminodiphenyl ether, Table 11, are unequal. Such results are not in accord with the expectations based upon .rr electron interactions. (IO) A. Streitwieser, Jr., and J. I.Brauman, "Supplemental Tables of Molecular Orbital Calculations", Pergamon Press, Oxford, 1965. (11) (a) M. J. S. Dewar and P. J. Grisdale, J . Am. Chem. Soc., 84, 3539 (1962);(b) P. R. Wells and W. Adcock, Aust. J . Chem., 18, 1365 (1965);(c) W. Adcock and M. J. S. Dewar, J . Am. Chem. Soc., 89, 379 (1967);(d) P. R. Wells, S. Ehrenson, and R. W. Taft, Prog. Phys. Org. Chem., 6, 147 (1968);(e) W. Adcock, P. D. Bettess, and S. Q. A. Rizvi, Aust. J . Chem., 23, 1921 (1970);(f) M. J. S. Dewar, R. Golden, and J. M. Harris, J . Am. Chem. Soc., 93, 4187 (1971);(9) W. Adcock, J. Alste, S. Q. A. Rizvi, and M. Aurangzeb, ibid., 98, 1701 (1976). (12) M. J. S.Dewar and A. J. Harget, Proc. R. SOC. London, Ser. A , 315, 457 (1970). (13) Correlation effects have an important influence on the magnitude of the contact shifts for the nuclei associated with the unconjugated positions. In this situation, there is no simple relationship between the shift measures for these nuclei and either the charge densities or the pR parameters. (14) (a) S. K. Dayal, S. Ehrenson, and R. W. Taft, J . Am. Chem. Soc., 94, 9113 (1972);(b) J. Fukunaga and R. W. Taft, ibid., 97, 1612 (1975). (c) Taft has suggested that the pR values most appropriate for a donor and a acceptor substituents may not be the same. Consequently,the anticipated relationship between pR and the contact shlfts may be negated for heteroatom bridging groups because the fluorine atom probe in NMR spectroscopy and the complexed amino substituent have opposite donor properties. Private communication. 15) The structure of benzophenone has been estblished by x-ray diffraction.'' The dihedral angle between the plane of the benzene ring and the plane of the carbonyl group is about 30'. LeFBvre had previously reached a similar conclusion for the molecule in solution." The delocalization of spin density between the aromatic rings would be appreclably attenuated in this propeller shaped molecule. 16) E. B. Fleischer, N. Sung, and S. Hawkinson, J . Phys. Chem., 72,
4311 (1968). 17) R. J. W. LeFBvre and J. D. Saxby, J . Chem. SOC.8,1064 (1966). 18) It is pertinent that the methylene bridge in fluorene is ineffective for the transmission of spin density from one ring to the other. This point is clearly established by the very small values of the contact chemical shlfts for 4-aminodiphenylmethane. We estimate that the contribution to the contact shift for the 7 hydrogen atom is -0.004 on the basis of the idea that the spln density in the position meta to the amino substituent is about 40% of the spin density in the position para to this substituent. (19) Our observations differ from the results obtained by the Eaton group.' is 2 compared to 1.2for the amines. They report that p,:-lpW I t may be that this is a serious discrepancy or it may be that the Diphenylyl derivative of the tropone iminate adopts a conformatlon which is unfavorable for the transmission of spin density between the phenyl nuclei.
