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A carbon-13 study of complexes of organonitriles with shift reagents
JOURNAL
OF MAGNETIC
RESONANCE
14,194-201
(19%)
A Carbon-13 Study of Complexesof Organonitriles with Shift Reagents* JUDITH A. YOUNG AND JEANETTE G. GRASSELLI Research Department, The Standard Oil Company (Ohio), Cleveland, Ohio 44218 AND
M. RITCHEY
WILLIAM
Case Western Reserve University, CleveIand, Ohio 44106 Received December 10, 1973 Carbon-13 magnetic resonance studies on a group of organonitriles containing an ether as well as a nitrile donor site showed large contact shift interactions with Eu(FOD)+ Due to steric hindrance effects in the oxynitriles, the nitrile was the preferred site of complexing. Contact shifts were also experienced by the carbons alpha to the nitrile, and qualitative estimates of the contributions from the pseudocontact and contact interactions for various carbons were made. The r3C studies supported conclusions on the choice of complex site in these heterofunctional molecules which were drawn from previous proton data. INTRODUCTION
It has been recently reported that organonitriles, although generally considered weak donors, do form complexes with paramagnetic shift reagents (I). The fluorinated shift reagents Eu(FOD), and Pr(FOD)3, which are stronger Lewis acids than the DPM chelates, were far more effective in generating shifts for spectral simplification. In the previous study, proton NMR data indicated that for difunctional molecules such as 7hydroxyheptanenitrile, both the hydroxy and the nitrile group do complex with the shift reagent and the measured gradients were normal. For a series of oxynitriles, the data indicated that the ether site, known to be a stronger donor than the nitrile (2), is in fact preferred over the nitrile group in complexing, but steric effects in both the ligand and the shift reagent are quite important and do influence the choice of complex site. In 4-phenoxybutyronitrile, the nitrile is the only significant site of bonding because of the bulky phenyl ring adjacent to the ether oxygen. This is true even using Eu(DPM), as the chelate where the central metal atom is far more accessible than in the Eu(FOD),. In 3-methoxy- and 3-ethoxypropionitrile, both the ether and nitrile sites are involved in the bonding. The ether is preferred using the Eu(DPM), chelate, but with the more bulky Eu(FOD), the nitrile site is definitely favored. The ether oxygens in the dioxydinitriles are available for complexing with the Eu(DPM)~, but with Eu(FOD), the nitrile is again preferred. The proton gradients for the groups adjacent to both the ether and nitrile sites in the complexes of these oxynitriles with Eu(DPM)~ are surprisingly low as compared to reported values (2). * Presented at the Central May 13-15, 1973.
Regional
Copyright 0 1974 by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain
Meeting, 194
American
Chemical
Society, Cleveland,
Ohio,
ORGANONITRILE
COMPLEXES
WITH
SHIFT
REAGENTS
195
Carbon-13 magnetic resonance studies were conducted to further elucidate the conclusions from proton data. It was hoped to gain more definitive results on steric and geometric factors which are determinate in choice of a complex site and also to obtain a better understanding of the nature of the complexation. EXPERIMENTAL
Apparatus
Proton NMR spectra were obtained on a Varian XL-loo-12 spectrometer operating at 30°C probe temperature and using tetramethylsilane (TMS) as an internal standard. Spectra were recorded at various sweep widths and times in order to facilitate accurate measurement of chemical shifts and to better observe splitting patterns for interpretation. Fluorine-19 spectra were obtained on the XL-loo-12 operating at 23.5 kG and 94.077 MHz. The proton signal from the chloroform solvent was used as the lock signal for field stabilization. All lgF chemical shifts are referenced to trifluoroacetic acid external reference and 5-mm sample tubes were used. Carbon-13 spectra were recorded using 12-mm sample tubes on a Varian XL-loo-15 pulsed spectrometer operating at 23.5 kG and 25.1 MHz. Probe temperature was 33°C. The instrument was controlled by a Varian 620/L 16K computer which also performed the Fourier transform operations. All proton couplings were removed by white noise decoupling. The deuterium signal from deuterated chloroform was used for field/ frequency lock. Sweep widths were varied. All chemical shifts are referenced to tetramethylsilane (TMS) internal standard, and accuracy is believed to be f2-3 Hz. Operating conditions varied from 400 to 2000 transients with an acquisition time of 0.6-l set with a 3-set pulse delay in some cases. For some samples, an exponential weighting time constant of 0.2-0.8 set was applied to the accumulated free induction decay before Fourier transformation. An rf pulse width of 20-40 psec, which corresponds to tipping angles of about 45-90”, was used. Models of Eu(FOD), and Eu(DPM), shift reagents and of the difunctional substrates were built using Framework Molecular Models produced by Prentice Hall, Inc. Reagents
The lanthanide shift reagents were purchased from Norell Chemical Company, New Jersey. The reagents were stored in a desiccator. The shift reagents used were tris(2,2,6,6tetramethyl-3,5-heptanedionato)europium (III) [Eu(THD),], also known as tris(dipivalomethanato)europium [Eu(DPM),], and [tris(l ,l, 1,2,2,3,3-heptafluoro-7,7dimethyl-4,6-octanedione)europium (III)] Eu(FOD),. Procedure
Spectra of the nitriles, all reagent-grade chemicals, were obtained in deuterated chloroform, chloroform, or in carbon tetrachloride. Chloroform-d was initially passed over 4-A molecular sieves to remove impurities which may decompose the complex. Concentrations of ligand were in the range I-4 M. Higher concentrations were required for 13C spectra than for the previous proton studies (1) but proton data was also obtained at higher concentrations for 4-phenoxybutyronitrile, 3-ethoxypropionitrile,
196
YOUNG,
GRASSELLI
AND
RITCHEY
3-methoxypropionitrile, and 3,3-tetramethylenedioxydipropionitrile with Eu(FOD),. The proton results at higher concentrations showed exactly the same trends in gradients as low-concentration experiments, and so concentration effects were not considered significant. Shift reagent was added incrementally to nitrile solutions except for lgF experiments. For these, the reverse procedure was used: i.e., the Eu(FOD), was dissolved in CHCl, at 0.09-0.11 Mconcentration. The ligand was then added incrementally to yield mole ratios of 0.05X1.2. The mole ratio refers to moles of shift reagent/mole of ligand. Linear graphs were obtained by least-squares fitting of the data. Standard deviations were typically 0.01-l. Slopes (gradients) and the y intercepts (extrapolated chemical shift of the free ligand) were also obtained from this least-squares program. DISCUSSION
Carbon-13 data for the oxynitriles is summarized in Table 1. The most striking observation is the large negative gradient obtained for the nitrile carbons in all four heterofunctional molecules. These results cannot be interpreted solely in terms of a pseudocontact interaction. A pseudocontact mechanism predicts that the site of complexation (assumed to be primarily the nitrile carbon based on proton data) would have the largest shift with attenuation down the chain. Table 1 shows that the nitrile carbon gradient does have the largest magnitude in all TABLE EFFECT
1
OF Eu(FOD)~ ON CARBON-13 RESONANCES OF HETEROFUNCTIONAL MOLECULES
Compound
Concentration of ligand (Ml
Carbons
Gradients
4-Phenoxybutyronitrile
1.09
A B C D
4.7 10.4 3.7 40.0
3-Methoxypropionitrile CHaOCH&H&N A B C D
2.03
A B C D
19.5 22.5 8.7 -24.8
3-Ethoxypropionitrile CH3CH20CHZCH&N A B CDE
2.45
A B C D E
5.4 11.8 16.7 7.0 -34.2
3,3-Tetramethylenedioxydipropionitrile NCCHICH20CH2CH&H&H~OCH&H&ZN EA B C D
0.94
A B C D E
2.4 6.1 4.0 3.0 -15.5
ORGANONITRILE
COMPLEXES
WITH
SHIFT
REAGENTS
197
cases, but in every case the shift is negative. Another significant aspect of the results is the alternation of the shifts from low to high magnitudes down the chain. Clearly, contact effects are involved and must be invoked to explain the deviations of the gradients from those predicted by the pseudocontact model. Contact interactions are, in fact, often seen in rr systems (3) and were demonstrated using acetone as a model ligand. Acetone plus Eu(FOD), was studied with both 13C and lH NMR. The gradients are shown in Table 2. The proton gradient for the alpha methyl is about 17, and this compares well with the reported values for ketones (2). Carbon-13 TABLE
EFFECTOF Eu(FOD)~
2
ON ACETONE RESONANCES 0
A
Concentration
B Gradients
of ligand (Ml 4.3 3.1
Proton
Carbon
17.38 A 13.3
B -31.9
gradients were of about the same order of magnitude as the proton gradients. The carbon labeled B, which is the carbonyl carbon, shows a negative shift for the gradient and thus indicates a contact shift (4) rather than pseudocontact interaction. The alpha carbon, labeled A, is apparently also experiencing some contact effect because its gradient is smaller in magnitude than would be expected from distance effects alone. This reflects a mixture of pseudocontact and contact effects in the alpha carbon. From this experiment, it was apparent that contact effects occur not only for the carbonyl carbon but even in the carbons alpha to the complex site. It was expected that a carbon of a nitrile would show a similar contact effect, and thus the observed results for nitriles were not entirely unpredicted. Johnson et al. (5) have reported that of the lanthanide chelates Eu(FOD), produces the largest contact contribution. With the DPM chelate, the contact effect is much less pronounced. Both 4-phenoxybutyronitrile and 3-methoxypropionitrile were run with Eu(DPM),, and the results are shown in Table 3. They do verify that the contact effect is less with Eu(DPM),. Models of the Eu(FOD), and Eu(DPM), were built in order to obtain some insight into the conformation and packing around the central metal atom in these chelates. The model assumes a coordination number of six and octahedral bonding around the central europium ion. The ionic radius of Eu and Van der Waals’ radii and covalent radii of carbon and hydrogen were utilized in the model construction. From the model of Eu(FOD),, it is clear that the groups are crowded about the central metal so that it is difficult for a bulky ligand to enter into the coordination sphere. Thus, end-on bonding
198
YOUNG,GRASSELLL AND RITCHEY TABLE 3 EFFECT
OF SHIET REAGENTS
ON HETEROFUNCTIONAL
MOLECULES
Gradients Compound 4-Phenoxybutyronitrile 0
Shift Reagent Eu(FOD)s
OCH2CH2CH&N ABCD
4-Phenoxybutyronitrile
Eu(DPM)a
Proton A B D C A
1.75 2.30 4.04
B
0.72 0.69
C
1.04
D 3-Methoxypropionitrile CH30CHJJH2CN A B CD
Eu(FOD),
3-Methoxypropionitrile
Eu(DPM),
A
B C D A B C D
5.56 6.37 6.16
Carbon 4.7 10.4 -40.0 3.7
1.5 3.7 2.6 -3.9 19.5 22.5 a.7 -24.8
13.15
17.7
12.40 7.16
16.7 7.2 0.7
with a nitrile is very facile and should be preferred over an ether oxygen as a complex site when Eu(FOD), is the chelate. In the Eu(DPM)~ chelate, there is relatively less crowding around the central metal than in the fluorinated chelate, allowing the ether oxygen to more easily complex with the less bulky DPM. The possibility of bonding both the ether and nitrile sites simultaneously to one central metal was found to be very unlikely due to the resulting ring strain, as demonstrated by the model. There are no compelling reasons to suggest that one ligand acts as a bridge between two metals, since the gradients were linear with concentration and their magnitudes were in the same range as monofunctional molecules, especially for the 3-methoxypropionitrile-Eu(DPM)3 complex. Again, models did not support the concept of bridging. The 19F spectrum of Eu(FOD), was recorded. The spectrum shows a singlet for the CF, alpha to the oxygen, a multiplet for the beta CF, group, and a triplet for the CF,. The fact that the alpha CF, group is a singlet indicates that its relaxation time is affected by the presence of the paramagnetic metal ion (or perhaps some rearrangement of the metal complex could be taking place, also). Several experiments were made where ligands including acetone, propionitrile, valeronitrile, and 3-ethoxypropionitrile were added incrementally, and resulting 19F spectra were recorded. Only small, but significant, shifts of the fluoride signal were observed. The 19F data suggest that the complexes consist of one Eu chelate and one ligand molecule. In order to help distinguish the pseudocontact and contact contributions, qualitative deductions were made to estimate the contact shift effect for carbons other than the nitrile carbon in 3-ethoxypropionitrile. The following assumptions were used. First, the
ORGANONITRILE
COMPLEXES
WITH
SHIFT
REAGENTS
199
methyl group labeled C, in Table 4 experiences no contact shift. Thus, its shift is purely pseudocontact and varies with the inverse cube of the distance from the center of the Eu III in accordance with Eq. [I] (6): A = K(3 cosz* 8 - 1) r31 ’
where 8 is the angle between the principal magnetic axis of the complex and the vector, of length r, connecting the metal atom and the nucleus under consideration. The second assumption is that the orientation of the magnetic axis in the Eu is parallel to the metal carbon axis. This is a reasonable assumption since Hawkes et al. (6) have TABLE CALCULATIONOF
4
CONTACTCONTRIBUTIONSFOR
CARBON-13'
CH$ZH20CH2CH&N A B CDE Carbon
k
A B C
4500
D
E
A obs (gradients)
AP (talc)
AC
5.4 11.8 16.7 7.0 -34.2
5.4 9.1 20.8 31.9 324.0
0 +2.4 4.1 -24.9 -358.2
’ Data for 3-ethoxypropionitrile
and Eu(FOD)~.
shown that the magnetic axis is essentially collinear with the metal oxygen bond in lanthanide shift reagent complexes with rigid alcohols such as borneols and isoborneols. Therefore, Eq. [l] reduces to Eq. [2]: A pseudo contact = %.
It is assumed that the observed shift will equal the shift due to both pseudocontact and contact effects as shown in Eq. [3]: A obs = A pseudo + A contact.
131
The constant k in Eq. [2] was evaluated for carbon A in the 3-ethoxypropionitrile Eu(FOD), chelate from its gradient and the distance as measured from the molecular model. For the measurement of r, an essentially extended “linear configuration” was assumed. Values of r were varied 1 A in each direction from the extended value, and no significant changes in the value of k were found. The pseudocontact interaction was calculated for the other four carbons using this value for k and are given in the A p column of Table 4. Using Eq. [3], the calculated pseudocontact values, and the observed gradients, the contact contributions, A c, were then obtained. From the values for A c, it is apparent that the contact contribution for the 13C resonances falls off rapidly with distance. Carbon B shows a positive value for A c, and this is most likely a result of the pseudocontact contribution from the limited bonding through the ether oxygen.
200
YOUNG,
GRASSELLI
AND
RITCHEY
Calculations of the pseudocontact and contact contributions for 3-methoxypropionitrile, 4-phenoxybutyronitrile, and 3,3-(tetramethylenedioxy)dipropionitrile are shown in Tables 5-7. The value of k is reasonably consistent in all cases except for 3-methoxypropionitrile. The average value for the contact contribution at the nitrile TABLE CALCULATIONOF
CONTACT
5
CONTRIBUTIONSFOR
CARBON-13'
OCHICH&H$N A B C D
Carbon
k
A B C D ED
A obs (gradients)
AP WC)
4.7 10.4
1.4
-40.0
215.8
-2.1 -3.5 -17.7 -255.8
3.0
3.0
0
13.9
3.7 3ooo
AC
21.4
a Data for 4-phenoxybutyronitrile and Eu(FOD)~. b k was evaluated on C’ of the ring. TABLE CALCULATION
OFCONTACT
6
CONTRIBUTIONSFOR
CARBON-13"
NCCH~CH~OCH~CH&H#ZH20CHZCH&H~CN EAB CD Carbon A B C D* E
k
2500
A obs (gradients)
AP
WC)
2.4
17.9
6.1
11.6
4.0 3.0
5.1 3.0
-15.5
179.9
AC
-15.5 -5.5
-1.1 0 -195.4
a Datafor3,3-(tetramethylenedioxy)dipropionitrileandEu(FOD),. b k was evaluated on carbon D.
from Tables 5-7 is -269.8. In the case of 3-methoxypropionitrile, Table 7, the calculated values fo’r both k and the resulting contact effect at the nitrile show marked deviations. This is very strong supporting evidence that in the case of 3-methoxypropionitrile there is interaction of the ether site with the shift reagent. For this molecule, the
carbon
ORGANONITRILE
COMPLEXES
WITH SHIFT REAGENTS
TABLE CALCULATION
OF CONTACT
201
7
CONTRIBLJTIONS
FOR CARBON-~?
CHa-O-CH2CH2CN A BCD Carbon
k
A B
9600
C
D
A obs (gradients)
19.5
19.5 22.5 8.7 -24.8
u Data for 3-methoxypropionitrile
AP (talc)
44.4 68.6 690.6
AC 0 -21.9 -59.9 -715.4
and Eu(FOD)~.
steric effects are minimal and the shift reagent will complex with the stronger donor site, the ether. If 13C data is interpreted in terms of both contact and pseudocontact effects, the results are reasonably consistent with the proton results. REFERENCES A. YOUNG, J. G. GRASSELLI, AND W. M. RITCHEY, Anal. Chem. 45, 1410 (1973). K. SANDERS AND D. H. WILLIAMS, J. Amer. Chem. Sot. 93,3,641 (1971). BRIGGS, F. A. HART, G. P. Moss, AND E. W. RANDALL, Chem. Commun. 364 (1971). WENKERT, D. W. COCHRAN, E. W. HAGAMAN, R. BURTON LEWIS, AND F. M. SCHELL, J. Amer. Chem. Sot. 93,23, 6271 (1971). 5. B. F. G. JOHNSON, J. LEWIS, P. MCARDLE, AND J. R. NORTON, Chem. Commun. 535 (1972). 6. G. E. HAWKES, D. LEIBFRITZ, D. W. ROBERTS, AND J. D. ROBERTS, J. Amer. Chem. Sot. 95,5,1659 (1973).
I. 2. 3. 4.
J. J. J. E.
OF MAGNETIC
RESONANCE
14,194-201
(19%)
A Carbon-13 Study of Complexesof Organonitriles with Shift Reagents* JUDITH A. YOUNG AND JEANETTE G. GRASSELLI Research Department, The Standard Oil Company (Ohio), Cleveland, Ohio 44218 AND
M. RITCHEY
WILLIAM
Case Western Reserve University, CleveIand, Ohio 44106 Received December 10, 1973 Carbon-13 magnetic resonance studies on a group of organonitriles containing an ether as well as a nitrile donor site showed large contact shift interactions with Eu(FOD)+ Due to steric hindrance effects in the oxynitriles, the nitrile was the preferred site of complexing. Contact shifts were also experienced by the carbons alpha to the nitrile, and qualitative estimates of the contributions from the pseudocontact and contact interactions for various carbons were made. The r3C studies supported conclusions on the choice of complex site in these heterofunctional molecules which were drawn from previous proton data. INTRODUCTION
It has been recently reported that organonitriles, although generally considered weak donors, do form complexes with paramagnetic shift reagents (I). The fluorinated shift reagents Eu(FOD), and Pr(FOD)3, which are stronger Lewis acids than the DPM chelates, were far more effective in generating shifts for spectral simplification. In the previous study, proton NMR data indicated that for difunctional molecules such as 7hydroxyheptanenitrile, both the hydroxy and the nitrile group do complex with the shift reagent and the measured gradients were normal. For a series of oxynitriles, the data indicated that the ether site, known to be a stronger donor than the nitrile (2), is in fact preferred over the nitrile group in complexing, but steric effects in both the ligand and the shift reagent are quite important and do influence the choice of complex site. In 4-phenoxybutyronitrile, the nitrile is the only significant site of bonding because of the bulky phenyl ring adjacent to the ether oxygen. This is true even using Eu(DPM), as the chelate where the central metal atom is far more accessible than in the Eu(FOD),. In 3-methoxy- and 3-ethoxypropionitrile, both the ether and nitrile sites are involved in the bonding. The ether is preferred using the Eu(DPM), chelate, but with the more bulky Eu(FOD), the nitrile site is definitely favored. The ether oxygens in the dioxydinitriles are available for complexing with the Eu(DPM)~, but with Eu(FOD), the nitrile is again preferred. The proton gradients for the groups adjacent to both the ether and nitrile sites in the complexes of these oxynitriles with Eu(DPM)~ are surprisingly low as compared to reported values (2). * Presented at the Central May 13-15, 1973.
Regional
Copyright 0 1974 by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain
Meeting, 194
American
Chemical
Society, Cleveland,
Ohio,
ORGANONITRILE
COMPLEXES
WITH
SHIFT
REAGENTS
195
Carbon-13 magnetic resonance studies were conducted to further elucidate the conclusions from proton data. It was hoped to gain more definitive results on steric and geometric factors which are determinate in choice of a complex site and also to obtain a better understanding of the nature of the complexation. EXPERIMENTAL
Apparatus
Proton NMR spectra were obtained on a Varian XL-loo-12 spectrometer operating at 30°C probe temperature and using tetramethylsilane (TMS) as an internal standard. Spectra were recorded at various sweep widths and times in order to facilitate accurate measurement of chemical shifts and to better observe splitting patterns for interpretation. Fluorine-19 spectra were obtained on the XL-loo-12 operating at 23.5 kG and 94.077 MHz. The proton signal from the chloroform solvent was used as the lock signal for field stabilization. All lgF chemical shifts are referenced to trifluoroacetic acid external reference and 5-mm sample tubes were used. Carbon-13 spectra were recorded using 12-mm sample tubes on a Varian XL-loo-15 pulsed spectrometer operating at 23.5 kG and 25.1 MHz. Probe temperature was 33°C. The instrument was controlled by a Varian 620/L 16K computer which also performed the Fourier transform operations. All proton couplings were removed by white noise decoupling. The deuterium signal from deuterated chloroform was used for field/ frequency lock. Sweep widths were varied. All chemical shifts are referenced to tetramethylsilane (TMS) internal standard, and accuracy is believed to be f2-3 Hz. Operating conditions varied from 400 to 2000 transients with an acquisition time of 0.6-l set with a 3-set pulse delay in some cases. For some samples, an exponential weighting time constant of 0.2-0.8 set was applied to the accumulated free induction decay before Fourier transformation. An rf pulse width of 20-40 psec, which corresponds to tipping angles of about 45-90”, was used. Models of Eu(FOD), and Eu(DPM), shift reagents and of the difunctional substrates were built using Framework Molecular Models produced by Prentice Hall, Inc. Reagents
The lanthanide shift reagents were purchased from Norell Chemical Company, New Jersey. The reagents were stored in a desiccator. The shift reagents used were tris(2,2,6,6tetramethyl-3,5-heptanedionato)europium (III) [Eu(THD),], also known as tris(dipivalomethanato)europium [Eu(DPM),], and [tris(l ,l, 1,2,2,3,3-heptafluoro-7,7dimethyl-4,6-octanedione)europium (III)] Eu(FOD),. Procedure
Spectra of the nitriles, all reagent-grade chemicals, were obtained in deuterated chloroform, chloroform, or in carbon tetrachloride. Chloroform-d was initially passed over 4-A molecular sieves to remove impurities which may decompose the complex. Concentrations of ligand were in the range I-4 M. Higher concentrations were required for 13C spectra than for the previous proton studies (1) but proton data was also obtained at higher concentrations for 4-phenoxybutyronitrile, 3-ethoxypropionitrile,
196
YOUNG,
GRASSELLI
AND
RITCHEY
3-methoxypropionitrile, and 3,3-tetramethylenedioxydipropionitrile with Eu(FOD),. The proton results at higher concentrations showed exactly the same trends in gradients as low-concentration experiments, and so concentration effects were not considered significant. Shift reagent was added incrementally to nitrile solutions except for lgF experiments. For these, the reverse procedure was used: i.e., the Eu(FOD), was dissolved in CHCl, at 0.09-0.11 Mconcentration. The ligand was then added incrementally to yield mole ratios of 0.05X1.2. The mole ratio refers to moles of shift reagent/mole of ligand. Linear graphs were obtained by least-squares fitting of the data. Standard deviations were typically 0.01-l. Slopes (gradients) and the y intercepts (extrapolated chemical shift of the free ligand) were also obtained from this least-squares program. DISCUSSION
Carbon-13 data for the oxynitriles is summarized in Table 1. The most striking observation is the large negative gradient obtained for the nitrile carbons in all four heterofunctional molecules. These results cannot be interpreted solely in terms of a pseudocontact interaction. A pseudocontact mechanism predicts that the site of complexation (assumed to be primarily the nitrile carbon based on proton data) would have the largest shift with attenuation down the chain. Table 1 shows that the nitrile carbon gradient does have the largest magnitude in all TABLE EFFECT
1
OF Eu(FOD)~ ON CARBON-13 RESONANCES OF HETEROFUNCTIONAL MOLECULES
Compound
Concentration of ligand (Ml
Carbons
Gradients
4-Phenoxybutyronitrile
1.09
A B C D
4.7 10.4 3.7 40.0
3-Methoxypropionitrile CHaOCH&H&N A B C D
2.03
A B C D
19.5 22.5 8.7 -24.8
3-Ethoxypropionitrile CH3CH20CHZCH&N A B CDE
2.45
A B C D E
5.4 11.8 16.7 7.0 -34.2
3,3-Tetramethylenedioxydipropionitrile NCCHICH20CH2CH&H&H~OCH&H&ZN EA B C D
0.94
A B C D E
2.4 6.1 4.0 3.0 -15.5
ORGANONITRILE
COMPLEXES
WITH
SHIFT
REAGENTS
197
cases, but in every case the shift is negative. Another significant aspect of the results is the alternation of the shifts from low to high magnitudes down the chain. Clearly, contact effects are involved and must be invoked to explain the deviations of the gradients from those predicted by the pseudocontact model. Contact interactions are, in fact, often seen in rr systems (3) and were demonstrated using acetone as a model ligand. Acetone plus Eu(FOD), was studied with both 13C and lH NMR. The gradients are shown in Table 2. The proton gradient for the alpha methyl is about 17, and this compares well with the reported values for ketones (2). Carbon-13 TABLE
EFFECTOF Eu(FOD)~
2
ON ACETONE RESONANCES 0
A
Concentration
B Gradients
of ligand (Ml 4.3 3.1
Proton
Carbon
17.38 A 13.3
B -31.9
gradients were of about the same order of magnitude as the proton gradients. The carbon labeled B, which is the carbonyl carbon, shows a negative shift for the gradient and thus indicates a contact shift (4) rather than pseudocontact interaction. The alpha carbon, labeled A, is apparently also experiencing some contact effect because its gradient is smaller in magnitude than would be expected from distance effects alone. This reflects a mixture of pseudocontact and contact effects in the alpha carbon. From this experiment, it was apparent that contact effects occur not only for the carbonyl carbon but even in the carbons alpha to the complex site. It was expected that a carbon of a nitrile would show a similar contact effect, and thus the observed results for nitriles were not entirely unpredicted. Johnson et al. (5) have reported that of the lanthanide chelates Eu(FOD), produces the largest contact contribution. With the DPM chelate, the contact effect is much less pronounced. Both 4-phenoxybutyronitrile and 3-methoxypropionitrile were run with Eu(DPM),, and the results are shown in Table 3. They do verify that the contact effect is less with Eu(DPM),. Models of the Eu(FOD), and Eu(DPM), were built in order to obtain some insight into the conformation and packing around the central metal atom in these chelates. The model assumes a coordination number of six and octahedral bonding around the central europium ion. The ionic radius of Eu and Van der Waals’ radii and covalent radii of carbon and hydrogen were utilized in the model construction. From the model of Eu(FOD),, it is clear that the groups are crowded about the central metal so that it is difficult for a bulky ligand to enter into the coordination sphere. Thus, end-on bonding
198
YOUNG,GRASSELLL AND RITCHEY TABLE 3 EFFECT
OF SHIET REAGENTS
ON HETEROFUNCTIONAL
MOLECULES
Gradients Compound 4-Phenoxybutyronitrile 0
Shift Reagent Eu(FOD)s
OCH2CH2CH&N ABCD
4-Phenoxybutyronitrile
Eu(DPM)a
Proton A B D C A
1.75 2.30 4.04
B
0.72 0.69
C
1.04
D 3-Methoxypropionitrile CH30CHJJH2CN A B CD
Eu(FOD),
3-Methoxypropionitrile
Eu(DPM),
A
B C D A B C D
5.56 6.37 6.16
Carbon 4.7 10.4 -40.0 3.7
1.5 3.7 2.6 -3.9 19.5 22.5 a.7 -24.8
13.15
17.7
12.40 7.16
16.7 7.2 0.7
with a nitrile is very facile and should be preferred over an ether oxygen as a complex site when Eu(FOD), is the chelate. In the Eu(DPM)~ chelate, there is relatively less crowding around the central metal than in the fluorinated chelate, allowing the ether oxygen to more easily complex with the less bulky DPM. The possibility of bonding both the ether and nitrile sites simultaneously to one central metal was found to be very unlikely due to the resulting ring strain, as demonstrated by the model. There are no compelling reasons to suggest that one ligand acts as a bridge between two metals, since the gradients were linear with concentration and their magnitudes were in the same range as monofunctional molecules, especially for the 3-methoxypropionitrile-Eu(DPM)3 complex. Again, models did not support the concept of bridging. The 19F spectrum of Eu(FOD), was recorded. The spectrum shows a singlet for the CF, alpha to the oxygen, a multiplet for the beta CF, group, and a triplet for the CF,. The fact that the alpha CF, group is a singlet indicates that its relaxation time is affected by the presence of the paramagnetic metal ion (or perhaps some rearrangement of the metal complex could be taking place, also). Several experiments were made where ligands including acetone, propionitrile, valeronitrile, and 3-ethoxypropionitrile were added incrementally, and resulting 19F spectra were recorded. Only small, but significant, shifts of the fluoride signal were observed. The 19F data suggest that the complexes consist of one Eu chelate and one ligand molecule. In order to help distinguish the pseudocontact and contact contributions, qualitative deductions were made to estimate the contact shift effect for carbons other than the nitrile carbon in 3-ethoxypropionitrile. The following assumptions were used. First, the
ORGANONITRILE
COMPLEXES
WITH
SHIFT
REAGENTS
199
methyl group labeled C, in Table 4 experiences no contact shift. Thus, its shift is purely pseudocontact and varies with the inverse cube of the distance from the center of the Eu III in accordance with Eq. [I] (6): A = K(3 cosz* 8 - 1) r31 ’
where 8 is the angle between the principal magnetic axis of the complex and the vector, of length r, connecting the metal atom and the nucleus under consideration. The second assumption is that the orientation of the magnetic axis in the Eu is parallel to the metal carbon axis. This is a reasonable assumption since Hawkes et al. (6) have TABLE CALCULATIONOF
4
CONTACTCONTRIBUTIONSFOR
CARBON-13'
CH$ZH20CH2CH&N A B CDE Carbon
k
A B C
4500
D
E
A obs (gradients)
AP (talc)
AC
5.4 11.8 16.7 7.0 -34.2
5.4 9.1 20.8 31.9 324.0
0 +2.4 4.1 -24.9 -358.2
’ Data for 3-ethoxypropionitrile
and Eu(FOD)~.
shown that the magnetic axis is essentially collinear with the metal oxygen bond in lanthanide shift reagent complexes with rigid alcohols such as borneols and isoborneols. Therefore, Eq. [l] reduces to Eq. [2]: A pseudo contact = %.
It is assumed that the observed shift will equal the shift due to both pseudocontact and contact effects as shown in Eq. [3]: A obs = A pseudo + A contact.
131
The constant k in Eq. [2] was evaluated for carbon A in the 3-ethoxypropionitrile Eu(FOD), chelate from its gradient and the distance as measured from the molecular model. For the measurement of r, an essentially extended “linear configuration” was assumed. Values of r were varied 1 A in each direction from the extended value, and no significant changes in the value of k were found. The pseudocontact interaction was calculated for the other four carbons using this value for k and are given in the A p column of Table 4. Using Eq. [3], the calculated pseudocontact values, and the observed gradients, the contact contributions, A c, were then obtained. From the values for A c, it is apparent that the contact contribution for the 13C resonances falls off rapidly with distance. Carbon B shows a positive value for A c, and this is most likely a result of the pseudocontact contribution from the limited bonding through the ether oxygen.
200
YOUNG,
GRASSELLI
AND
RITCHEY
Calculations of the pseudocontact and contact contributions for 3-methoxypropionitrile, 4-phenoxybutyronitrile, and 3,3-(tetramethylenedioxy)dipropionitrile are shown in Tables 5-7. The value of k is reasonably consistent in all cases except for 3-methoxypropionitrile. The average value for the contact contribution at the nitrile TABLE CALCULATIONOF
CONTACT
5
CONTRIBUTIONSFOR
CARBON-13'
OCHICH&H$N A B C D
Carbon
k
A B C D ED
A obs (gradients)
AP WC)
4.7 10.4
1.4
-40.0
215.8
-2.1 -3.5 -17.7 -255.8
3.0
3.0
0
13.9
3.7 3ooo
AC
21.4
a Data for 4-phenoxybutyronitrile and Eu(FOD)~. b k was evaluated on C’ of the ring. TABLE CALCULATION
OFCONTACT
6
CONTRIBUTIONSFOR
CARBON-13"
NCCH~CH~OCH~CH&H#ZH20CHZCH&H~CN EAB CD Carbon A B C D* E
k
2500
A obs (gradients)
AP
WC)
2.4
17.9
6.1
11.6
4.0 3.0
5.1 3.0
-15.5
179.9
AC
-15.5 -5.5
-1.1 0 -195.4
a Datafor3,3-(tetramethylenedioxy)dipropionitrileandEu(FOD),. b k was evaluated on carbon D.
from Tables 5-7 is -269.8. In the case of 3-methoxypropionitrile, Table 7, the calculated values fo’r both k and the resulting contact effect at the nitrile show marked deviations. This is very strong supporting evidence that in the case of 3-methoxypropionitrile there is interaction of the ether site with the shift reagent. For this molecule, the
carbon
ORGANONITRILE
COMPLEXES
WITH SHIFT REAGENTS
TABLE CALCULATION
OF CONTACT
201
7
CONTRIBLJTIONS
FOR CARBON-~?
CHa-O-CH2CH2CN A BCD Carbon
k
A B
9600
C
D
A obs (gradients)
19.5
19.5 22.5 8.7 -24.8
u Data for 3-methoxypropionitrile
AP (talc)
44.4 68.6 690.6
AC 0 -21.9 -59.9 -715.4
and Eu(FOD)~.
steric effects are minimal and the shift reagent will complex with the stronger donor site, the ether. If 13C data is interpreted in terms of both contact and pseudocontact effects, the results are reasonably consistent with the proton results. REFERENCES A. YOUNG, J. G. GRASSELLI, AND W. M. RITCHEY, Anal. Chem. 45, 1410 (1973). K. SANDERS AND D. H. WILLIAMS, J. Amer. Chem. Sot. 93,3,641 (1971). BRIGGS, F. A. HART, G. P. Moss, AND E. W. RANDALL, Chem. Commun. 364 (1971). WENKERT, D. W. COCHRAN, E. W. HAGAMAN, R. BURTON LEWIS, AND F. M. SCHELL, J. Amer. Chem. Sot. 93,23, 6271 (1971). 5. B. F. G. JOHNSON, J. LEWIS, P. MCARDLE, AND J. R. NORTON, Chem. Commun. 535 (1972). 6. G. E. HAWKES, D. LEIBFRITZ, D. W. ROBERTS, AND J. D. ROBERTS, J. Amer. Chem. Sot. 95,5,1659 (1973).
I. 2. 3. 4.
J. J. J. E.