The relaxation time of several ferrocene derivatives was measured, and the internal rotation was discussed. For almost all the derivatives, the degree of the internal rotation was constant in spite of the different molecular weights. However, for (triphenylmethyl)ferrocene, the rotation of the unsubstituted ring would be slower due to the bulkiness of the substituent. Furthermore, the derivatives that have a hydroxyl- or acetyl group on the substituent were also discussed. Their rotation would be influenced by the location of these substituents.
One of the interesting features of ferrocenes is the rotation of the cyclopentadienyl (Cp) ring around the Cp-Fe-Cp bond axis. On the unsubstituted ferrocene in solution, the two Cp rings freely rotate around the bond axis [
For the 1,1'-disubstituted ferrocenes, many studies have mainly used the dipole moment measurement. The two Cp rings in 1,1'-diacetyl-or 1,1'-benzoyl- ferrocene freely rotate [
The authors reported the Cp ring rotation on the following derivatives. The Cp rings of 1,1'-dit-butylferrocene freely rotate, but the rings of 1,1'-bis(triphe- nylmethyl)ferrocene having bulky substituents were restricted, and the cis-like conformation was very unstable [
On the other hand, a study of the monosubstituted ferrocenes was done by a longitudinal relaxation time NMR method instead of the dipolar moment [
The ferrocene derivatives used in this study were synthesized by well-known methods [
The longitudinal relaxation time was measured using JEOL A-400 and ECS-400 spectrometers.NMR samples consisted of 0.5 ml of solution in 5 mm o.d. tubes. After degassing by passing Ar gas, the spectra were recorded at ambient temperature. Longitudinal relaxation times values were measured under proton-noise- decoupling conditions by the inversion recovery method.
The partial molar volumes were measured using a Lipkin-Devison type pycnometer. The ferrocene derivatives were dissolved in benzene, and the solution was
Scheme 1. Synthetic routes of the ferrocene derivatives.
added in the pycnometer. The concentration of the solutions was 0.5 - 3.0 wt%. The pycnometer was sintered in a thermostatic tank at 30˚C, and the volume of solution were measured. From this volume and weight of the solution, the partial molar volumes were calculated.
The relaxation time is a parameter for the mobility of molecule or its part. The ferrocene Cp rings rotate around the Cp-Fe-Cp bond axis. The Cp carbon relaxation times are influenced by the degree of the Cp ring mobility. However, the degree contains several mobility modes; rotation, vibration, translation, and so on. For discussion of the internal rotation, the relaxation times of the two Cp rings must be compared.
For comparing the substituted and unsubstituted Cp rings, the values of the 1'- and β-positions were used. Because the 1'-position carbon on the unsubstituted Cp ring has hydrogen atoms on both side carbons, the β-position carbon on the substituted Cp ring is similar.
The longitudinal relaxation time of the Cp group carbon is summarized in
Compound | Substituent | Longitudinal relaxation time (T1)/s | 1'/β | ||
---|---|---|---|---|---|
1' | α | β | |||
1 | ethyl | 14.50 | 10.30 | 10.70 | 1.36 |
2 | t-butyl | 13.50 | 8.58 | 9.06 | 1.49 |
3 | pentadecyl | 6.14 | 3.72 | 3.87 | 1.59 |
4 | octadecyl | 5.23 | 3.13 | 3.55 | 1.47 |
5 | benzyl | 9.91 | - | 6.84 | 1.45 |
6 | diphenylmethyl | 6.38 | 4.60 | 4.69 | 1.36 |
7 | triphenylmethyl | 6.97 | 3.12 | 3.12 | 2.23 |
8 | o-methylphenyl | 8.44 | 5.59 | 5.78 | 1.46 |
9 | o-hydroxyphenyl | 5.66 | 2.83 | 4.24 | 1.33 |
10 | o-(hydroxymethyl)phenyl | 7.08 | 3.03 | 3.86 | 1.83 |
11 | 1-hydroxyethyl | 9.51 | 7.21 | 7.59 | 1.25 |
12 | 1-hydroxypentyl | 7.13 | 3.35 | 4.26 | 1.67 |
13 | 1-hydroxydecyl | 7.32 | 3.61 | 4.61 | 1.59 |
14 | 1-hydroxypentadecyl | 6.11 | 2.89 | 4.00 | 1.53 |
15 | 1-hydroxybenzyl | 6.46 | ? | 3.31 | 1.95 |
aSolvent: benzene, bTemperature: ambient.
Compound | Substituent | Partial molar volume/cm3/mol |
---|---|---|
ethyl | 66.7 | |
2 | t-butyl | 62.5 |
3 | pentadecyl | 64.0 |
4 | octadecyl | 61.7 |
5 | benzyl | 89.7 |
6 | diphenylmethyl | 93.0 |
7 | triphenylmethyl | 111.3 |
8 | o-methylphenyl | 91.5 |
9 | o-hydroxyphenyl | 122.3 |
10 | o-(hydroxymethyl)phenyl | 115.4 |
11 | 1-hydroxyethyl | 94.7 |
12 | 1-hydroxypentyl | 94.6 |
13 | 1-hydroxydecyl | 82.3 |
14 | 1-hydroxypentadecyl | 73.6 |
15 | 1-hydroxybenzyl | 149.5 |
aSolvent: benzene, bTemperature: ambient.
The relationship between the molecular weight and 1'/β value is shown in
The relationship between the molecular weight and the partial molar volume is indicated in
These results would be interpreted as follows. The bulky triphenylmethyl group exists to hang over unsubstituted Cp ring protons. As a result, the space in which the solvent is difficult to approach is produced (
rotation of the Cp group is not prevented by the solvent, thus the 1'/β value becomes high. For diphenylmethyl group, a conformation in which the phenyl groups do not exist near the unsubstituted Cp proton is possible by the rotation around Cp-Ph bond axis. Such a conformation would result in the smaller 1'/β value and middle partial molar volume (
For the alkyl- and arylferrocenes, the 1'/β value was mostly fixed except for (triphenylmethyl)ferrocene (
When (o-hydroxyphenyl)ferrocene and [o-(hydroxymethyl)phenyl]ferrocene were compared, the former showed a low 1'/β value. In (o-hydroxyphenyl)fer- rocene, a strong OH-d type hydrogen bond is formed, so that the hydroxyl group exists near the unsubstituted Cp ring [
As mentioned in a previous paper [
structure in which a metal cationis incorporated between the carbonyl oxygen and iron atom. In this section, the effect on the relaxation time by such a chelate ring formation is discussed. Here, acetonitrile-d3 was employed as the solvent to dissolve the metal salts, but the solubility is low. Therefore, the 1H relaxation time was used for the following discussion. The longitudinal relaxation time of the Cp group proton is summarized in
(o-Acetylphenyl)ferrocene alone showed a 1.77 1'/β value. In the presence of Ca2+, which does not form a strong chelate, the value did not significantly change. However, Al3+ forming a strong chelate structure (
By comparing the relaxation time of substituted and unsubstituted Cp rings, the internal rotation of substituted ferrocenes was discussed. For almost all the
Entry | Metal ion | Longitudinal relaxation time (T1)/s | 1'/β | |
---|---|---|---|---|
1' | β | |||
1 | none | 6.97 | 3.93 | 1.77 |
2 | Ca2+ | 4.72 | 2.55 | 1.85 |
3 | Al3+ | 3.36 | 2.54 | 1.32 |
aSolvent: acetonitrile-d3, bTemperature: ambient.
derivatives, the degree of the internal rotation was constant in spite of the different molecular weights. However, for (triphenylmethyl)ferrocene, the rotation of the unsubstituted ring would be slower. Furthermore, such a slow internal rotation is found for some derivatives that have a hydroxyl- or acetyl group on the substituent. These slow rotations would result in the existence of space in which the solvent is difficult to approach.
Okada, Y., Yamamoto, S., Namba, Y., Masuda, T. and Sakamoto, K. (2016) Internal Rotation of Cyclopentadienyl Rings in Ferrocene Derivatives. Spectral Analysis Reviews, 4, 41-48. http://dx.doi.org/10.4236/sar.2016.44004