Ferrocene-Containing Pseudorotaxanes in Crystals: Aromatic Interactions with Hammett Correlation

Single crystals of pseudorotaxanes, [(FcCH2NH2CH2Ar)(DB24C8)][PF6] (DB24C8 = dibenzo[24]crown-8, Fc = Fe(C5H4)(C5H5), Ar = -C6H3-3,4-Cl2, -C6H3-3,4-F2, -C6H4-4-F, -C6H4-4-Cl, -C6H4-4-Br, -C6H3-3-F-4-Me, -C6H4-4-I) and [(FcCH2NH2CH2C6H4-4-Me)(DB24C8)][Ni(dmit)2] (dmit = 1,3-dithiole-2,4,5-dithiolate), were obtained from solutions containing DB24C8 and ferrocenylmethyl(arylmethyl)ammonium. X-ray crystallographic analyses of the pseudorotaxanes revealed that the aryl ring of the axle moiety and the catechol ring of the macrocyclic component were at close centroid distances and parallel or tilted orientation. The structures with parallel aromatic rings showed correlation of the distances between the centroids to Hammett substituent constants of the aryl groups.

In the last few decades, we have investigated structures and properties of the crystalline pseudorotaxanes of DB24C8 and ferrocenylmethyl(arylmethyl)ammonium, [(FcCH2NH2CH2C6H4-4-Me)(DB24C8)][EF6] (E = P, As) [44][45][46][47][48][49][50]. The pseudorotaxanes caused the crystalline phase transition upon heating and photo-irradiation. Related crystalline supramolecules were reported to exhibit new stimulus-response behavior [51][52][53][54]. Scheme 1 shows two structures of the pseudorotaxane of DB24C8 with ferrocenylmethyl(4-methylphenylmethyl)ammonium in the crystals. The pseudorotaxane with the PF6 − counter anion was supported by multiple N-H⋯O hydrogen bonds, π-π interaction between the 4-methylphenyl group and a catechol group, and C(Cp)-H⋯π interaction between the ferrocenyl group and the other catechol group (Scheme 1a, α-form). The distance and angle of the catechol ring and p-methylphenyl ring was determined to be 3.71 Å and 6.2°, respectively. The pseudorotaxane with AsF6 − anion preferred the structure with C⋯H-π interaction between the 4-methylphenyl group of the axle component and a catechol group (Scheme 1b, β-form). Heating crystals of [(FcCH2NH2CH2C6H4-4-Me)(DB24C8)]PF6 above 128 °C caused thermal crystalline phase transition from α-form to β-form. Recent studies revealed that the crystalline phase transition temperature of the crystals was influenced largely by size of the counter anions [47,50]. In solution, the pseudorotaxanes of DB24C8 and benzyl(arylmethyl)ammonium, [(ArCH2(PhCH2)NH2)(DB24C8)][PF6] have been reported to show different stabilities, depending on the substituents on the aromatic groups of the axle component [55]. However, there have been no reports on relevance of the crystalline structures of such (pseudo)rotaxanes to the substituents of the aromatic group of the arylmethylammonium axle component. Here we report the crystal structures of pseudorotaxanes composed of DB24C8 and ferrocenylmethyl(arylmethyl)ammonium and show the effect of the aryl group on the molecular structures of the pseudorotaxanes. This study focuses on relative positions and orientation of the neighbouring aromatic groups of the axle and cyclic components in the α-form pseudorotaxane crystals.

Results and Discussion
Mixing DB24C8 with ferrocenylmethyl(arylmethyl)ammonium in solution caused crystal growth of the corresponding pseudorotaxanes. Their structures were determined by X-ray crystallography (vide infra). The reaction of DB24C8 with ferrocenylmethyl(arylmethyl)ammonium formed the corresponding pseudorotaxanes, 1a-1g, as crystals, as shown in eq 1. Similar pseudorotaxanes with Ph, C6H4-4-Me, and C6H4-4-OMe groups in the axle component, 1h-1i, were reported previously [46][47][48] In solution, the pseudorotaxanes of DB24C8 and benzyl(arylmethyl)ammonium, [(ArCH 2 (PhCH 2 )NH 2 )(DB24C8)][PF 6 ] have been reported to show different stabilities, depending on the substituents on the aromatic groups of the axle component [55]. However, there have been no reports on relevance of the crystalline structures of such (pseudo) rotaxanes to the substituents of the aromatic group of the arylmethylammonium axle component. Here we report the crystal structures of pseudorotaxanes composed of DB24C8 and ferrocenylmethyl(arylmethyl)ammonium and show the effect of the aryl group on the molecular structures of the pseudorotaxanes. This study focuses on relative positions and orientation of the neighbouring aromatic groups of the axle and cyclic components in the α-form pseudorotaxane crystals.

Results and Discussion
Mixing DB24C8 with ferrocenylmethyl(arylmethyl)ammonium in solution caused crystal growth of the corresponding pseudorotaxanes. Their structures were determined by Xray crystallography (vide infra). The reaction of DB24C8 with ferrocenylmethyl(arylmethyl) ammonium formed the corresponding pseudorotaxanes, 1a-1g, as crystals, as shown in Equation (1). Similar pseudorotaxanes with Ph, C 6 H 4 -4-Me, and C 6 H 4 -4-OMe groups in the axle component, 1h-1i, were reported previously [46][47][48]   Pseudorotaxane 1c has two aromatic groups A and B with the orientation close to orthogonal, suggesting C-H⋯π interaction between the aromatic groups A and B, as shown in Figure 1b. The structure is similar to 1i above the crystalline phase transition temperature and belongs to β-form in Scheme 1. Pseudorotaxane 1i-Ni contains the 4methylphenyl group of the axle component and a phenylene group of a catechol group in parallel fashion, as shown in Figure 1c. Previous crystallographic studies of 1h, 1i, and 1j showed that multiple C-H⋯F interactions between DB24C8 and PF6 − impart the relative stability of α-to the β-form [46,47,50]. Figure 2 depicts interaction of the cationic pseudorotaxane with [Ni(dmit)2] − anion of 1i-Ni, which differs largely from that of pseudorotaxane 1i with PF6 − anions. Figure 1a shows structure of pseudorotaxane 1d with a chlorophenyl group in the axle component. The ammonium hydrogens, H1 and H2, are at close positions to the oxygen atoms of DB24C8 (N1-H1· · · O2: 2.216 Å, N1-H1· · · O3: 2.240 Å, N1-H2· · · O1: 2.537 Å, N1-H2· · · O8: 2.368 Å), suggesting N-H· · · O hydrogen bonds. The cyclopentadienyl ligand forms a C-H· · · π interaction (3.09 Å) with a C 6 H 4 ring of DB24C8. The distance between the centroid of phenylene ring A of the axle component and that of the catechol ring B of DB24C8 (d, Å) and the angle formed by the aromatic planes (θ, • ) are 3.70 Å and 5.26 • , respectively. Thus, the structure of pseudorotaxane 1d belongs to the α-form of Scheme 1.   Pseudorotaxane 1c has two aromatic groups A and B with the orientation close to orthogonal, suggesting C-H⋯π interaction between the aromatic groups A and B, as shown in Figure 1b. The structure is similar to 1i above the crystalline phase transition temperature and belongs to β-form in Scheme 1. Pseudorotaxane 1i-Ni contains the 4methylphenyl group of the axle component and a phenylene group of a catechol group in parallel fashion, as shown in Figure 1c. Previous crystallographic studies of 1h, 1i, and 1j showed that multiple C-H⋯F interactions between DB24C8 and PF6 − impart the relative stability of α-to the β-form [46,47,50]. Figure 2 depicts interaction of the cationic pseudorotaxane with [Ni(dmit)2] − anion of 1i-Ni, which differs largely from that of pseudorotaxane 1i with PF6 − anions. Pseudorotaxane 1c has two aromatic groups A and B with the orientation close to orthogonal, suggesting C-H· · · π interaction between the aromatic groups A and B, as shown in Figure 1b. The structure is similar to 1i above the crystalline phase transition temperature and belongs to β-form in Scheme 1. Pseudorotaxane 1i-Ni contains the 4methylphenyl group of the axle component and a phenylene group of a catechol group in parallel fashion, as shown in Figure 1c. Previous crystallographic studies of 1h, 1i, and 1j showed that multiple C-H· · · F interactions between DB24C8 and PF 6 − impart the relative stability of αto the β-form [46,47,50].  [Ni(dmit)2] (1i-Ni). The IR peaks of the symmetric and asymmetric vibration of ammonium N-H bonds of 1a-1d, 1f, 1g (3065-3080, 3166-3195 cm −1 ) were observed at lower wavenumbers than those of starting ammonium, 2a-2g (3233-3236 and 3262-3268 cm −1 ), due to the hydrogen bonding between the ammonium and oxygen atoms of DB24C8. The pseudorotaxane crystals of 1i in α-form and in β-form were reported to have different conformation of the axle molecule and co-conformation of the axle and macrocyclic molecules (orientation of the axle molecule within the pseudorotaxane framework) [50]. The rotaxanes in α-form (1a, 1d-1i) and those in β-form (1b, 1c, 1h) in Table  1 showed different wavenumbers of the IR peaks due to νas vibrations of the NH2 group, 3184 cm −1 on average for 1a and 1d-1i and 3161 cm −1 on average for 1b, 1c, and 1h. The distances between N1 and O2 atoms of 1a and 1d-1i, 3.079 Å on average, were longer than those of 1b, 1c, and 1h (2.960 Å on average). Thus, these spectroscopic and structural parameters relating to the N-H⋯O hydrogen bonds differ clearly between the crystals of α-form and those of β-form. Table 2 summarizes relative positions and orientations of two aromatic planes A and B of the pseudorotaxanes in the crystalline state. The two aromatic planes of α-form were almost parallel in the structures with tilt angles, in   N-H bonds of 1a-1d, 1f, 1g (3065-3080, 3166-3195 cm −1 ) were observed at lower wavenumbers than those of starting ammonium, 2a-2g (3233-3236 and 3262-3268 cm −1 ), due to the hydrogen bonding between the ammonium and oxygen atoms of DB24C8. The pseudorotaxane crystals of 1i in α-form and in β-form were reported to have different conformation of the axle molecule and co-conformation of the axle and macrocyclic molecules (orientation of the axle molecule within the pseudorotaxane framework) [50]. The rotaxanes in α-form (1a, 1d-1i) and those in β-form (1b, 1c, 1h) in Table 1 showed different wavenumbers of the IR peaks due to ν as vibrations of the NH 2 group, 3184 cm −1 on average for 1a and 1d-1i and 3161 cm −1 on average for 1b, 1c, and 1h. The distances between N1 and O2 atoms of 1a and 1d-1i, 3.079 Å on average, were longer than those of 1b, 1c, and 1h (2.960 Å on average). Thus, these spectroscopic and structural parameters relating to the N-H· · · O hydrogen bonds differ clearly between the crystals of α-form and those of β-form. Table 2  Both values are much smaller than the corresponding values of pseudorotaxanes in β-form, 1b, 1c, and 1h.  Figure 3 shows Hammett plots of structural parameters of crystalline pseudorotaxanes with α- form, 1a, 1d-1g and 1i-1j. The distances between centroids (Figure 3a) and dihedral angles (Figure 3b) of aromatic groups A and B were plotted against the Hammett constants, σ, of A [56]. Hammett constants of disubstituted aromatic group in 1a and 1f were calculated by assuming additivity of Hammett constants [57,58]. Linear relationships were observed for d and θ values to Hammett constants, and σ-values were calculated as −0.21 and −4.1, respectively. Thus, aromatic group A with a larger σ-value was positioned at a closer position to aromatic group B with a smaller dihedral angle. Coefficients of determination of the plots in Figure 3a,b were similar (R 2 = 0.76 and 0.73), suggesting that parameters d and θ were correlated with each other. Attempts to plot averaged distances between aromatic planes of A and B to Hammett constants resulted in lower correlation than that between d and θ. The two aromatic planes were almost parallel in the structures but had slight differences in the structural parameters. The d and θ values of pseudorotaxanes were increased by electron-donating substituents (negative σ values) of the terminal aryl group of the axle component.   Figure 3 shows Hammett plots of structural parameters of crystalline pseudorotaxanes with α- form, 1a, 1d-1g and 1i-1j. The distances between centroids (Figure 3a) and dihedral angles (Figure 3b) of aromatic groups A and B were plotted against the Hammett constants, σ, of A [56]. Hammett constants of disubstituted aromatic group in 1a and 1f were calculated by assuming additivity of Hammett constants [57,58]. Linear relationships were observed for d and θ values to Hammett constants, and σ-values were calculated as −0.21 and −4.1, respectively. Thus, aromatic group A with a larger σ-value was positioned at a closer position to aromatic group B with a smaller dihedral angle. Coefficients of determination of the plots in Figure 3a,b were similar (R 2 = 0.76 and 0.73), suggesting that parameters d and θ were correlated with each other. Attempts to plot averaged distances between aromatic planes of A and B to Hammett constants resulted in lower correlation than that between d and θ The two aromatic planes were almost parallel in the structures but had slight differences in the structural parameters. The d and θ values of pseudorotaxanes were increased by electron-donating substituents (negative σ values) of the terminal aryl group of the axle component.   such as 1d, 1e, 1g, 1i, and 1j, were plotted against R + constants in order to estimate the contribution of the resonance effect for the Hammett plots in Figure 3. Centroid distance of pseudorotaxane 1j with OMe group at the 4-position of A (3.779 Å) is much longer than other pseudorotaxanes with Cl, Br, I, and Me groups. The coefficient of determination obtained from the plots of the five pseudorotaxanes is high (R 2 = 0.95). This indicates that resonance effect of the aromatic group A is significant among the mono-substituted aromatic groups. These results indicate that the pseudorotaxanes bearing mono-and disubstituted aromatic group A showed that the electronic nature of A influenced the relative positions and orientations of the aryl groups A and B.
The centroid distance (d) and dihedral angle (θ) of 1i-Ni (3.665 Å and 4.59 • ) were smaller than those of 1i with PF 6 anion (3.710 Å and 6.20 • ). Such effects of the counter anion on the structure of cationic pseudorotaxane are ascribed to the different co-conformation of the pseudorotaxanes caused by the counter anions (vide supra) [50].
Theoretical studies compared three possible geometries for the aromatic interactions, slipped-parallel, parallel, and perpendicular ones (Scheme 2). Tsuzuki et al. calculated stabilities of the benzene dimers as the function of distance (d) and angles (θ) between them and reported the optimized position for slipped paralleled conformation (d = 3.5 Å, ∆G • = −2.48 kcal mol −1 (at the CCSD(T) level)) which is more stable than the parallel type interaction (∆G • = −1.48 kJ mol −1 ) and similar to C-H· · · π interaction (∆G • = −2.46 kJ mol −1 ) [59][60][61][62]. Thus, the energy differences among the possible interacted structures are small. Recently, parallel stacking of the aromatic rings (Scheme 2b) was found in the crystals of polyhedral oligomeric silsesquioxane (POSS) derivatives, although it was considered to be less stable than the others [63]. The combination of two aromatic rings at close positions was known to influence stability of their π-π stacking. As a further important factor, donor-acceptor interaction was known to stabilize the aromatic interaction significantly.
contribution of the resonance effect for the Hammett plots in Figure 3. Centroid distance of pseudorotaxane 1j with OMe group at the 4-position of A (3.779 Å ) is much longer than other pseudorotaxanes with Cl, Br, I, and Me groups. The coefficient of determination obtained from the plots of the five pseudorotaxanes is high (R 2 = 0.95). This indicates that resonance effect of the aromatic group A is significant among the mono-substituted aromatic groups. These results indicate that the pseudorotaxanes bearing mono-and disubstituted aromatic group A showed that the electronic nature of A influenced the relative positions and orientations of the aryl groups A and B.
The centroid distance (d) and dihedral angle (θ) of 1i-Ni (3.665 Å and 4.59°) were smaller than those of 1i with PF6 anion (3.710 Å and 6.20°). Such effects of the counter anion on the structure of cationic pseudorotaxane are ascribed to the different coconformation of the pseudorotaxanes caused by the counter anions (vide supra) [50].
Theoretical studies compared three possible geometries for the aromatic interactions, slipped-parallel, parallel, and perpendicular ones (Scheme 2). Tsuzuki et al. calculated stabilities of the benzene dimers as the function of distance (d) and angles (θ) between them and reported the optimized position for slipped paralleled conformation (d = 3.5 Å , ΔG° = −2.48 kcal mol −1 (at the CCSD(T) level)) which is more stable than the parallel type interaction (ΔG° = −1.48 kJ mol −1 ) and similar to C-H⋯π interaction (ΔG° = −2.46 kJ mol −1 ) [59][60][61][62]. Thus, the energy differences among the possible interacted structures are small. Recently, parallel stacking of the aromatic rings (Scheme 2b) was found in the crystals of polyhedral oligomeric silsesquioxane (POSS) derivatives, although it was considered to be less stable than the others [63]. The combination of two aromatic rings at close positions was known to influence stability of their π−π stacking. As a further important factor, donor-acceptor interaction was known to stabilize the aromatic interaction significantly.  indicate that the C 6 H 3 -3,4-Cl 2 ring of 1a and the catechol ring of DB24C8 is closer and less tilted than those of the C 6 H 3 -4-OMe ring of 1i and the catechol ring of DB24C8 because of stronger donor-acceptor interaction in the former system. Stoddart, Williams, and their co-workers investigated a full series of pseudorotaxanes composed of DB24C8 and bis(arylmethyl)ammonium in the solid state and in solution. They observed a clear relationship between the stability constants for the pseudorotaxane and the electron donating ability of the substituents of the aryl groups of the axle components in CDCl 3 and CD 3 CN-CDCl 3 [55]. Higher stability of pseudorotaxanes possessing aryl groups with electron-withdrawing groups, such as NO 2 and COOH groups, at the para position can be attributed to the aromatic interaction between the axle and macrocyclic components. Although direct estimation of the aromatic interaction was difficult in the solutions, the results in the solid of this study state are related to the relative stability of the pseudorotaxanes in the solution.  Stoddart, Williams, and their co-workers investigated a full series of pseudorotaxanes composed of DB24C8 and bis(arylmethyl)ammonium in the solid state and in solution. They observed a clear relationship between the stability constants for the pseudorotaxane and the electron donating ability of the substituents of the aryl groups of the axle components in CDCl3 and CD3CN-CDCl3 [55]. Higher stability of pseudorotaxanes possessing aryl groups with electron-withdrawing groups, such as NO2 and COOH groups, at the para position can be attributed to the aromatic interaction between the axle and macrocyclic components. Although direct estimation of the aromatic interaction was difficult in the solutions, the results in the solid of this study state are related to the relative stability of the pseudorotaxanes in the solution.