For the synthesis of the double-motorized porphyrin cage compoundZn3 (Supporting Information, Scheme S1) we followed the same strategy as applied for the synthesis of mono-motorized cageZn2 .25 Compound Zn3 was obtained as a mixture of six stereoisomers, i.e. three diastereomeric sets of enantiomers. The different diastereomers Zn3a (first eluted, C2-symmetric), Zn3b (second eluted, non-symmetric), and Zn3c (third eluted, C2-symmetric), see Figure 2, were separated by conventional silica gel column chromatography. We resolved the enantiomers of each diastereomeric set in order to be able to obtain more information about the structures of the individual compounds by chiroptical spectroscopy. Resolution was achieved by preparative chiral HPLC using a Chiralpak IE column (Supporting Information, Figures S1–S12). The ECD spectra of all the enantiomers of double-motorized compound Zn3 were compared to the ECD spectra of the corresponding single-motorized compound Zn2 (Figure 2A–B), especially with respect to the Cotton effect of the molecular motor absorption band at 250–300 nm and the Cotton effect of the Soret band at 420–430 nm. Isomers with a positive Cotton effect of the Soret band were assigned (+), and those with a negative Cotton effect of the Soret band were assigned (−). The relative stereochemistry of Zn3awas identical to that of Zn2a , i.e. for both compounds the motor and Soret CD bands had the same sign. Compounds Zn3cand Zn2balso displayed similarities, in the sense that the motor and Soret CD bands had opposite signs. Because
FIGURE 2 Spectroscopic comparison of motorized porphyrin cage compoundsZn2 and Zn3 . ECD spectra (CH2Cl2, 298 K) of: (A) (+)-Zn2and (+)-Zn3 ; (B) (−)-Zn2 and (−)-Zn3 . (C) Partial 1H NMR spectra (500 MHz, CDCl3/CD3CN, 1:1, v/v, 298 K) ofZn2 and Zn3 . See Figure 1 for the proton assignments. (D) Key NOEs (red arrows) of motor protons, spatial orientation, and absolute configuration of isomers (+)-Zn3a , (+)-Zn3b , and (+)-Zn3c . The descriptors (R,P) and/or (S,M) represent the point chirality and helicity of the molecular motor substituents in their stable configuration, and the descriptor Sp (or Rp, not shown in this Figure) represents the planar chirality of the porphyrin cage.
of the presence of two motor substituents, the magnitude of the Cotton effect of the motor substituents of Zn3a and Zn3c was doubled with respect to that of Zn2a and Zn2b . The non-symmetric compound Zn3b is different from all the other compounds, since it contains two molecular motor substituents with opposite chirality and, therefore, the motor CD band was absent. Comparison of the CD spectra of Zn3 with those of Zn2allowed the determination of the absolute configuration of each isomer (Supporting Information, Table S1). The 1H NMR spectra of Zn2 and Zn3 gave additional evidence for the above-mentioned differences and similarities between the compounds (Figure 2C). In our previous work on single-motorized cage Zn2we showed that the chemical shifts of motor protons H-a and H-b were indicative of the presence or absence of intramolecular interactions between the naphthalene part of the motor and the cavity of the porphyrin cage.25 The presence of such interactions in compound Zn2b was evidenced by a strong shielding effect on protons H-a and H-b , which is caused by the parallel displaced π-π stacking of the naphthalene part of the motor and the porphyrin. In contrast, this shielding effect was not present in Zn2a and also not in theC 2-symmetric compound Zn3a , confirming their similarity. It was observed, however, for one of the two motors of non-symmetric compound Zn3b , and for both motors ofC 2-symmetric compound Zn3c . These observations indicate similarities between the three-dimensional structures of single- and double-motorized porphyrin cages Zn2and Zn3 . We conclude, therefore, that Zn3a contains two molecular motors, which both are loosely connected to the cage with no significant intramolecular interaction with the cage cavity. On the other hand, Zn3b is equipped with one ‘loose’ molecular motor, and one molecular motor that is intramolecularly ‘bound’ to the cavity. Finally, compound Zn3c carries two ‘bound’ molecular motors. 2D ROESY experiments confirmed the three-dimensional structures of the different isomers of Zn3 (Figure 2D). The differences in steric impediment between the ‘loose’ and ‘bound’ molecular motors may translate into different affinities of the cage compounds for guest molecules, i.e. it is expected that one or more ‘bound’ motor substituents will inhibit binding of a guest in the cavity of hostZn3 .
We reported previously that porphyrin cage compoundsZn1 24 and Zn2 25display a high affinity for viologen guests. In this study, we evaluated the binding affinities of the different isomers of Zn3 for viologen guests V4V6 by fluorescence titrations (Table 1). During these titrations, the porphyrins were excited at their main Q-band (λex = 555 nm) to avoid in situ excitation of the molecular motor substituents. The experiments revealed that isomer Zn3a (two loose motors) binds all viologen guests with a roughly two times higher association constant (K a) than isomer Zn3b (one loose motor, one bound motor). In turn, isomer Zn3b binds the guests with a roughly two times higher K a-value than isomerZn3c (two bound motors). Apparently, bound motor substituent progressively impede the binding of the viologen guest. The absolute values of the association constants depend on the N -substituents of the viologen. We reason that for all isomers of Zn3 the binding of n -pentyl viologen V6 is more sterically demanding than the binding of methyl viologen V4 , resulting in lower K a-values. In contrast, benzyl viologenV5 forms the strongest complexes with all the isomers ofZn3 , which we attribute to the ability of the benzyl functions of this guest to form additional π-π interactions with the molecular motor substituents of the host. The binding studies and trends presented in Table 1 only represent the behavior of the motorized porphyrin cages in their stable ground states. Isomerization of the molecular motor substituents to the metastable states results in opposite relative stereochemistries, and therefore it may also result in different binding behavior.
TABLE 1 Association constants (Ka in M−1) and Gibbs free energies (ΔG0 in kJ/mol, in brackets) for the complexes of the isomers of porphyrin cageZn3 with viologen guests V4V6(CHCl3/CH3CN, 1:1, v/v, 298 K).