Influence of the Solvent and Metal Center on Supramolecular Chirality Induction with Bisporphyrin Tweezer Receptors. Strong Metal Modulation of Effective Molarity Values

We describe the synthesis of a bisporphyrin tweezer receptor 1 center dot H-4 and its metalation with Zn(II) and Rh(III) cations. We report the thermodynamic characterization of the supramolecular chirality induction process that takes place when the metalated bisporphyrin receptors coordinate to enantiopure 1,2-diaminocyclohexane in two different solvents, toluene and dichloromethane. We also performed a thorough study of several simpler systems that were used as models for the thermodynamic characterization of the more complex bisporphyrin systems. The initial complexation of the chiral diamine with the bisporphyrins produces a 1:1 sandwich complex that opens up to yield a simple 1:2 complex in the presence of excess diamine. The CD spectra associated with the 1:1 and 1:2 complexes of both metalloporphyrins, 1 center dot Zn-2 and 1 center dot Rh-2, display bisignate Cotton effects when the chirogenesis process is studied in toluene solutions. On the contrary, in dichloromethane solutions, only 1 center dot Zn-2 yields CD-active 1:1 and 1:2 complexes, while the 1:2 complex of 1 center dot Rh-2 is CD-silent. In both solvents, porphyrin 1 center dot Zn-2 features a stoichiometrically controlled chirality inversion process, which is the sign of the Cotton effect of the 1:1 complex is opposite to that of the 1:2 complex. In contrast, porphyrin 1 center dot Rh-2 affords 1:1 and 1:2 complexes in toluene solutions with the same sign for their CD couplets. interestingly, in both solvents, the signs of the CD couplets associated with the 1:1 sandwich complexes of 1 center dot Zn-2 and 1 center dot Rh-2 are opposite. The amplitudes of the CD couplets are higher for 1 center dot Zn-2 than for 1 center dot Rh-2. This observation is in agreement with 1 center dot Rh-2 having a smaller extinction coefficient than 1 center dot Zn-2. We performed DFT-based calculations and assigned molecular structures to the 1:1 and 1:2 complexes that explain the observed signs for their CD couplets. Unexpectedly, the quantification of the thermodynamic stability of the two metallobisporphyrin/diamine 1:1 sandwich complexes revealed the existence of interplay between effective molarity values (EM) and the strength of the intermolecular interaction (K-m; N center dot center dot center dot Zn or N center dot center dot center dot Rh) used in their assembly. The EM for the N center dot center dot center dot Rh(III) intramolecular interaction is 3 orders of magnitude smaller than that for the N center dot center dot center dot Zn(II) interaction, both of which are embedded in the same scaffold of the 1 center dot M-2 bisporphyrin receptor.