The ferrocene derivatives used in this study were synthesized by well-known methods [4] [6] [7] [8] . The compounds were identified by 1H NMR, IR, and mass spectra as described in the previous paper [4] [6] . The synthetic routes were shown in Scheme 1.

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 relationship between the molecular weight and 1'/β value is shown in Figure 1. (ο-Methylphenyl)ferrocene and benzylferrocene are isomers (molecular weight 276), and their 1'/β values were almost equal (Table 1 Compounds 5 and 8). This shows that the kind of substituent does not affect the rotation properties. The 1'/β values of almost all the derivatives containing these two derivatives were about 1.5. However, for (triphenylmethyl)ferrocene, the 1'/β value was higher (Figure 1). Based on this result, the unsubstituted Cp ring of this derivative would rotate faster than the rings of the other derivatives.

The relationship between the molecular weight and the partial molar volume is indicated in Figure 2. In this figure, the derivatives were classified into three groups. The first group is the alkylferrocenes, and the partial molar volumes are about 65 (Figure 2). The second group is the derivatives having phenyl groups, and their values are about 90 (Figure 2). The last one is (triphenylmethyl)fer- rocene, and its value is 110 (Figure 2). This classification shows that the derivatives possessing phenyl group(s) have larger volumes than the derivatives possessing no phenyl group. Especially the (triphenylmethyl)ferrocene’s volume is significantly large.

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 (Figure 3). Therefore, the

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 (Figure 2).

For the alkyl- and arylferrocenes, the 1'/β value was mostly fixed except for (triphenylmethyl)ferrocene (Figure 4). Of the ferrocene derivatives having a hydroxyl group, only (1-hydroxybenzyl)ferrocene showed a high value (Table 1 Compound 15). Furthermore, the derivative has a largerpartial molar volume than the other derivatives (Figure 5). In (1-hydroxybenzyl)ferrocene, the benzene ring is twisted by the formation of the OH-π type hydrogen bond [9] . Therefore, the substituent would produce a space in which the solvent is difficult to approach. On the other hand, (1-hydroxyalkyl)ferrocene has no such hydrogen bond, thus it would have similar values for 1'/β and the partial molar volume.

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 [6] . Therefore,the rotation of the Cp ring would be disturbed. On the other hand, [o-(hydroxymethyl)phenyl]ferro- cene does not have such a hydrogen bond, because the hydroxyl group and iron atom are too close.

As mentioned in a previous paper [10] , (o-acetylphenyl)ferrocene has a chelate

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 Table 3.

(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 (Figure 6) reduces the 1'/β value. These results would be due to the chelate structure in which the metal cation and carbonyl oxygen exist in the neighborhood of the unsubstituted Cp group. Therefore, the Cp ring rotation would be prevented in a manner similar to that for the (o-hydroxyphenyl)ferrocene mentioned in Section 3.2.